To be sure, it hasn't been used to describe male college students in any meaningful way. The term is exclusively reserved for female college students who attend(ed) college with men. Various etymological sources trace the first appearance of "co-ed" as a noun used to refer to a group of women eating among a group of men in Louisa May Alcott's 1886 book Jo's Boys.

Never saw girls eat such a lot. It proves that they ought not to study so much. Never liked co-ed," growled Dolly, much ruffled in spirit.

"So they do. 'Tis n't ladylike. Ought to be satisfied with an ice and a bit of cake, and eat it prettily. Don't like to see a girl feed. We hard-working men need it...."

Alcott was using the term as a noun to describe a woman or girl in a place that was coded as, or identified as, a male space, doing things that were seen as unfeminine or inappropriate for a girl or woman to do. The term was used sporadically by authors until the early 1900's when it became more firmly associated with higher education.

To back up a bit, college simply wasn't an option for women in America until Oberlin began to accepted women in 1835. Philosophically speaking, Oberlin was a deeply progressive, Christian college and when they referred to their female students, they were described as "scholars" just like the male students, or "young ladies." It's important to stress that Oberlin welcomed women shortly after it was founded (although women were kept from enrolling in some classes), and was considered radical for providing higher education to Black men and women. Basically, women were welcome from the beginning and they were not seen as intruding or encroaching on a male space. Despite Oberline's decision to open their doors to all women, Black and white, institutions of higher education would continue to be the domain of men (mostly white) for decades. Higher education at this time was focused on a classical curriculum, the concept of a "major" hadn't yet emerged. Men would study Latin, Greek, maths, logic, and some sciences with the purpose of becoming an educated man. Meanwhile, it was a common belief, regardless of class or race, that a woman's future role as a wife or mother needed nothing more than her innate skills and perhaps a high school education. There are exceptions but they were incredibly rare.

The mid-1800's and Reconstruction saw a rise in white girls' attendance at grammar and high schools, putting pressure on the system at large to provide for their education beyond high school. This pressure gave rise to the Seven Sisters (Barnard, Bryn Mawr, Mount Holyoke, Smith, Wellesley, Vassar, and Radcliffe). The colleges were located in the Northeast and while each one opened with a different focus and charge, they formed a network that paralleled the eight most prestigious men's colleges in the country, collectively known as The Ivy League (Brown, Columbia, Cornell, Dartmouth, Harvard, University of Pennsylvania, Princeton, and Yale). Like Oberlin, writing from Sisters' administrators and professors typically referred to their students as "young Misses" or "young ladies." The colleges were explicitly coded female and feminine, in some cases the lack of male students and a focus on privacy was a key selling point, but their identity wasn't generally defined in comparison to male students. With one key exception: Radcliffe.

Multiple women, all white, tried repeatedly to get into Harvard and rather than allow them to take classes with male students, Harvard administration, in 1879, elected to create a seperate, segregated learning space. The Harvard professors who led the courses were paid by a committee of Boston women, rather than by the college itself and despite the women's success, Harvard did not budge. After a decade of appeals and clear evidence that the women did just fine handling a classical curriculum while at the "Harvard Annex", it looked as if Harvard was about to bend but concerns that women at Harvard would fatally harm its reputation - and their own - endured. As a PR push in 1893, the Harvard Annex community opened their doors to local reporters. One paper, the Boston Port, ran a front page story by a woman reporter titled: A Harvard Annex Girl. Pretty Little Miss Freshmen and Her Week of Study. The Clubs and What They Do - A Historic Mansion - From Lunch-Room to Library -- Social Phases of College Life--Wrong Impression Corrected. From the article:

Harvard Annex girls are just like other girls - only prettier. They're just wise enough to be sensible, and just jolly enough not to be blue stockings and there isn't a large proportion of eyeglasses among them, either. Harvard Annex girls have nothing in common with their Harvard University brethren except the most important of all - the Harvard professors and the Harvard examinations. Little Miss Freshmen ... is a blonde, with her yellow hair parted demurely in the middle and started off into little tendrils of lovelocks either side, braided sedately behind and done up in a young-ladyfied knot. And her eyes are blue and her complexion is a warm-tinted pink and white, and she had dimples and good nature and good sense and all things which go to make up a typical Harvard Annex maid.

In case you're not picking up what they were putting down - it was important everyone knew being educated did not negatively impact their femininity. The PR campaign worked. Harvard bent the tiniest bit and agreed to support the founding of Radcliffe College. Students were required to take the same entrance exams as men applying to Harvard, took similar coursework, worked with professors who taught at both schools but did not receive a Harvard diploma. Radcliff students were segregated into a single-gender space; it was not a co-education setting. Instead, Harvard, Radcliff administrators, and the local press went to incredible lengths to infantilize the women who attended.

Although the average age was, generally speaking, the same at Harvard and Radcliffe, they were known as "Harvard Men" and "Radcliffe Girls." They were described as "Maids" or "Misses" and "young" virtually always preceded "Lady." People in the community - including the students themselves - went to great lengths to stress that an education did not negatively impact their marriage prospects. They were still nubile, still young, fair, and not too smart. They were still worth a man's time and attention. Articles praising the women for maintaining their beauty and making the "right" choice to end their studies once they got engaged were common in local papers.

It's difficult to stress just how many women wanted to create a life of their own choosing - and how much the male gatekeepers at colleges and universities wanted to stop them. The term "co-ed" was used to other the women who dared intrude on their domain. From the Santa Cruz Sentinel, 13 Nov 1904:

Another Rap for U.C. Co-Eds Professor George Holmes Howison, head of the department of philosophy at Berkeley and a man whose reputation in his particular field of scholarship is world-wide, has little sympathy with the co-ed in college...

Women after woman, Black and white, applied to various colleges and universities throughout the 1890's and early 1900's. Some were let in under very specific conditions - such as future high school teachers sitting in on chemistry lectures at Harvard. Otherwise, were allowed to attend a set "ladies" schedule, but barred from courses deemed "inappropriate." By 1912, though, the phrase "co-ed" was part of the vernacular. In a series of books published in 1912 and 1914, an author from Iowa detailed the adventures of a young woman attending college. The four books followed her college career and were called "A Freshman Co-ed" etc. The main character gets into scrapes and is described as, "a bright-haired little maiden with some very difficult problems to solve."

The use of "co-ed" to describe a female student served primarily one key role: to identify her outsider status. Unlike proper young ladies who went to the Sisters, she dared intrude on a male space. It didn't matter if there were a dozen young women, or hundreds, by 1930, a female college student at a mixed-gender college was known as a "co-ed." The term peeked in the 1940's, and became a mainstay of wedding announcements:

From the Indianapolis Star, 28 May 1944: All-American Pihos Weds Co-Ed at I.U. "The marriage of Pete Pihos, Indiana University All-American football end in 1943, to Dorothea U. Lansing, I.U. senior from Royal Oak..."

Even though some colleges went the other way, men returning from the war started attending what were previously small women-only teacher prep colleges, the term didn't apply to them. The use of the term became regional throughout the 1960's and 70's and was considered old-fashioned by the 1980's as women attended college at rates equal to or greater than men. Some colleges, especially public universities funded by state dollars, released guidance advising the college newspaper or professors to avoid the term.

It's difficult to say it's completely fallen out of favor as people were writing letters to the editor calling for it to be eliminated from usage as late as 2012. Searches with the keyword pull up results from articles and podcasts published in 2018. When it's used, though, in the modern era, the speaker generally wants to appeal to the same thing the Boston Port reporter wanted to appeal to in 1893; the woman being discussed is young, attractive, unmarried, and not too smart.