As I understand it, the word libertarian originated with the French anarchists in the 1700s, and was for hundreds of years used interchangeably or in conjunction with words like anarchist, socialist, and communist. How exactly did such a drastic change in meaning come about over the last few decades? Were there groups hundreds of years ago in Europe that used the word in the same way that it's used in America today, or is it really a new phenomenon?
This is actually something that has intrigued & irritated me for some time. Especially during election cycles in the U.S. when the word begins to see increased usage, often by people who neither understand what it means or care to develop a more nuanced understanding.
I have come to the understanding that this misappropriation isn't particularly new or 'organic'. Rather, it was the result of a deliberate act by right-wing political activists in America back in the late 1960's and early 1970's. Murray Rothbard wrote in his book The Betrayal of the American Right:
One gratifying aspect of our rise to some prominence is that, for the first time in my memory, we, 'our side,' had captured a crucial word from the enemy... 'Libertarians'... had long been simply a polite word for left-wing anarchists, that is for anti-private property anarchists, either of the communist or syndicalist variety. But now we had taken it over...
The term libertarian has not been used for the same thing over the years by different people. The term is still used in philosophy to describe the opposing viewpoint to philosophical determinism.
Although a definition of libertarianism is debated by many, libertarians emphasize legal rights to "negative" freedoms (e.g. the right not to be murdered), while usually recognizing no legal rights to "positive" freedoms (e.g. the right to receive healthcare).
In the United States, those that described themselves as libertarians used to see themselves as plain liberals. According to their viewpoint, the term was co-opted by the progressive left during the first half of the 20th century.
For example a whole book called "Liberalism" was written by Ludwig von Mises. Here you can find a libertarian writing during this transition period. However, he was a European coming to America and this book was originally in German. In the preface to the English translation he writes:
"In the United States 'liberal' means today a set of ideas and political postulates that in every regard are the opposite of all that liberalism meant to the preceding generations."
I give this example because its convenient he gives a direct reference to this transition. To quote an American I give you H.L. Mencken, an American Journalist and Literary Critic, describing his views:
"I am an extreme libertarian, and believe in absolute free speech. . . . I am against jailing men for their opinions, or, for that matter, for anything else."
This comes from "Letters of H.L. Mencken." It is telling he decided to use the term libertarian instead of liberal.
The term now used to describe the historical liberals is "classical liberalism." The modern left thinks of themselves as building on this tradition, which is a complex argument tangential to your question. Liberatarians would say that they are not building on this tradition. Whoever is correct is a matter of your viewpoint that is not something I am not addressing here.