Most people know that “gay” originally meant “happy/carefree” and came to refer to (usually, male) homosexuals, but not how or when the change occurred. When we study the etymology of the word, which is twelfth century in origin, it’s clear it had developed sexually suspect connotations as early as the 1500s. By the seventeenth century, its meaning of “carefree” had become specific to a kind of sexual looseness or immorality: a “gay woman” was a prostitute, a “gay man” was a womaniser, and a “gay house” was a brothel. Cities known for their liberal nightlife were described as gay — “gay Paris” — and unmarried men and women were often referred to as gay, in reference to them being sexually available (whether or not they exercised that availability). “Gay” could also refer to the act of sex itself.
In America, the expression “gay cat,” recorded as early as 1893, referred to young men who drifted from place to place and were noted for their lack of sexual discernment. There’s also evidence they were taken advantage of because of their age, and in some cases kept almost as slaves. Sociologists noted that “Homosexual practices are more common than rare in this group,” and in a 1933 dictionary of Underword & Prison Slang, “gay cat” was defined as a “homosexual boy.”
“A Gay Cat,” said he, “is a loafing laborer, who works maybe a week, gets his wages and vagabonds about hunting for another ‘pick and shovel’ job. Do you want to know where they got their monica [moniker] ‘Gay Cat’? See, Kid, cats sneak about and scratch immediately after chumming with you and then get gay. That’s why we call them ‘Gay Cats’.”
— Leon Ray Livingston, “Life and Adventures of A-no. 1,” 1910
It is almost certainly from “gay cat” that “gay” came to be synonymous with “homosexual,” starting around 1920-30, although as the word emerged in an underground subculture there’s no exact documentation to evidence when its meaning first changed. Some academics suggest it meant homosexual as early as the 1880s: Wildean scholars have studied his works and some believe he picked up “gay” — meaning “homosexual” — during his tour of America and brought it back to London; others point to his use of the word “earnest” and wonder if that, too, wasn’t a subcultural shorthand for “queer”. A prostitute caught up in the Cleveland Street Scandal of 1889 referred to queer men specifically as “gay.” The works of Noel Coward, the English playwright, have also been studied for their very specific use of the word “gay” to support the theory it emerged in the American theatrical scene at the turn of the twentieth century.
Bringing Up Baby, released in 1938, is the first motion picture to use the word gay in the sense of meaning queer: when Cary Grant’s character is asked why he’s wearing a feather boa, he replies, “Because I just went gay all of a sudden!”
By the 1960s, gay had become synonymous enough with homosexual for it to be used in that context in popular media, and a flurry of references in novels, TV shows, and films commenced in that decade.