I'm sure this is something most of us have never even thought about but it's quite interesting. As I was reading the book "Why Do Men Leave The Seat Up?" this question appeared and its one of the most interesting ones in the book.
"Long before the word "gay" had any connection to homosexuals, it meant "full of joy" or "brilliant and gaudy" (as in how a person was dressed). The word wasn't used to describe homosexuals until the late eighteen hundreds.
Around the turn of the 20th century, hobos traveled the continental U.S. via train cars, panhandling and providing menial tasks in return for spare change. A boy who was traveling companion of and an apprentice to a hobo was referred to as a "gey cat". When panhandling didn't get the job done, the young hobos would provide sexual favors to strangers of either gender in exchange for money. Furthermore, the nature of a gey cat's status as an apprentice was catamite, meaning that he provided sex to older hobos in return for being taught the vagabonding ropes.
Homosexual men called each other "gay" by the 1920's, and the term made its theatrical debut in 1929 in Noel Coward's operetta "Bitter Sweet" in the lyrics of the song "Green Carnation". The song alluded to Oscar Wilde, who was famously homosexual and always wore a green carnation. (In Wilde's Paris of the late nineteenth century, it was fashionable for a gay man to wear a green carnation in his lapel hole.)
Hollywood latched on to the trendy meaning of the word and brought it to a national audience in 1938. In the movie "Bringing Up Baby", Cary Grant's character is portrayed as effeminate, while his leading lady, Katherine Hepburn, is the overpowering and demanding one. The role reversal wasn't something that audiences of the era were used to seeing. In one scene, Grant's character has to wear women's clothing and says that he "just went gay, all of a sudden!"
"Gay" soon become an accepted antonym for heterosexual; it was the least offensive word that was available at the time. In fact, calling someone a "homosexual" was a slur of sorts. Up until 1973, homosexuality was listed as a clinically diagnosed mental illness in the Diagnostic and Statistical Manual Of Mental Disorders. "Queer", another common way to describe a homosexual person means "odd" and has derogatory undertones.
Although today the word "gay" can also mean "lame" and boring", thanks to the youth of the late 1990s, it is most widely used in the context of homosexuality. "Gay" is considered a politically correct way to describe homosexuals, but it can be offensive if misused. Here's an example: Acceptable: "Those gay men in the bar were rather stylishly dressed." Offensive: "Did you see the way those gay guys were dressed?"
The bottom line? Even though it's common, "gay" is not a word to be bandied about carelessly."
TO EACH IT'S OWN.