Gay (homosexual) and gay (happy)

Question

When did the main meaning of the word 'gay' shift from happy to homosexual? How did the meaning evolve, if there is a relation between the two?

Answer 1

The development is actually quite straightforward once it has been pointed out.

As your question indicates that you already know, 'gay' originally meant 'carefree' and 'happy'.

Over time, the liberalness of 'carefree' increased and by the 1600s, it was used to describe someone of promiscuous sexual proclivities. Womanisers, prostitutes and other such sexually liberal people were termed 'gay'.

The idea of sexuality within the word continued to become more liberal and sexual promiscuity came to be sexual obscurity and eventually, by C20 (yes, that recent) it was used to refer to homosexuals.

Now, that meaning has developed even further and the word 'gay' has taken on a negative connotation (since many oppose homosexuality) and can now be used almost interchangeably with the word 'bad', in certain youth cultures. i.e.: "I don't want that one; it's gay"

Answer 2

The New Oxford American Dictionary has:

USAGE.   Gay meaning ‘homosexual,’ dating back to the 1930s (if not earlier), became established in the 1960s as the term preferred by homosexual men to describe themselves. It is now the standard accepted term throughout the English-speaking world. As a result, the centuries-old other senses of gay meaning either ‘carefree’ or ‘bright and showy,’ once common in speech and literature, are much less frequent. The word gay cannot be readily used unselfconsciously today in these older senses without sounding old-fashioned or arousing a sense of double entendre, despite concerted attempts by some to keep them alive. Gay in its modern sense typically refers to men ( lesbian being the standard term for homosexual women), but in some contexts it can be used of both men and women.

Etymonline has more to say:

The word gay by the 1890s had an overall tinge of promiscuity — a gay house was a brothel. The suggestion of immorality in the word can be traced back at least to the 1630s, if not to Chaucer: “But in oure bed he was so fressh and gay / Whan that he wolde han my bele chose.”

Slang meaning “homosexual” (adj.) begins to appear in psychological writing late 1940s, evidently picked up from gay slang and not always easily distinguished from the older sense. The association with (male) homosexuality likely got a boost from the term gay cat, used as far back as 1893 in Amer.Eng. for "young hobo," one who is new on the road, also one who sometimes does jobs.

The "Dictionary of American Slang" reports that gay (adj.) was used by homosexuals, among themselves, in this sense since at least 1920.

Gay as a noun meaning "a (usually male) homosexual" is attested from 1971; in M.E. it meant "excellent person, noble lady, gallant knight," also "something gay or bright; an ornament or badge" (c. 1400)

^{(many citations omitted; see the link above for the full text)}

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For some personal research, Google ngram gives two interesting graphs:

Both graphs show a reversal in the trend of usage of gay in the late 1970s. Of course, those graphs are established from books, so written usage in other media and oral usage probably predate that in books.