Here's a breakdown of (1) the origin of the word; (2) the popularity of its usage over time, as seen by the saturation of its appearance in literature; and (3) Its most common meaning/use.

Origin

The word "pinko" was coined by Time magazine in 1925 as a variant on the noun and adjective pink, which had been used along with parlor pink since the beginning of the 20th century to refer to those of leftish sympathies, usually with an implication of "effeteness."

Source: Wikipedia article, "Pinko"

Usage Frequency

Here is a graphic of the usage of the term "pinko" in literature over time, since 1900. What you can see here is the growth of its usage, and what time "pinko" reached its most prominent usage.

Meaning/Use

[It's common usage came to] describe a person regarded as being sympathetic to communism, though not necessarily a Communist Party member. It has since come to be used, derogatorily, to describe anyone perceived to have leftist or socialist sympathies.

The term has its origins in the notion that pink is a lighter shade of red, a color associated with communism. Thus pink could describe a "lighter form of communism", purportedly promoted by supporters of socialism who were not themselves actual or "card carrying" communists. The term pinko has a pejorative sense, whereas 'pink' in this definition can be used in a purely descriptive sense, such as in the term pink tide.

Source: Wikipedia article, "Pinko"

EDIT: Added Usage Frequency graph, and broke apart Origin and Meaning/Use

It comes mainly from the 1950 - 1954 period that is often known as the "Red scare" period.

We owe this to Joseph McCarthy, a US senator who actively tried to purge USA from everyone remotely suspected of communist sympathies and claimed the US government was being complacent with leftists who were slowly infiltrating it and spying for Moscow.

Ensued a nationwide witch hunt where everyone not agreeing with McCarthy's ideas was either a Soviet agent provocateur or a weakling that did not have the balls to actually take the means to fight communism in the US.

Even though McCarthy is long gone, the idea that communism was an insult equivalent to traitor remained in the US.

It is worth noting that this meaning of "communist = traitors" is non-existant outside of the US, in Europe only the most extreme right wing politicians use this discourse and most European nations still have Communist Parties (even though they tend to not get a lot of votes generally) which would be unthinkable in the US.

If you are interested in the period feel free to read "The Politics of Fear : Joseph R. McCarthy and the Senate" by Robert Griffith.