Definition & Meaning of "three-letter man"in English

What is the origin of the term "three-letter man" and how is it used?

Three-letter man is a slang phrase that appeared in mid-20th-century English as an insult for a gay man. The term comes from the three letters in the word "gay," using the length of the word itself to label and mock someone's sexuality. Historically, it was derogatory, intended to belittle or shame men for being homosexual. Today it is largely historical and considered offensive; using it in modern conversation would almost always be insulting.