I'm going to attempt ... to answer it. It's not going to be a particularly good, satisfying answer, at least not to me. I'm only doing it (and inviting all those ... uh ... merry ... downvotes) on the chance that it might get someone thinking along the right lines ... and stimulate and inspire them sufficiently to come up with something better.
Before it became an ironic, and then altogether too serious, euphemism for "homosexual," (and later still a code word for "lame, weak"), the adjective "gay" denoted a light-to-considerable degree of promiscuity, both male and female; and before that only female promiscuity. It was used facetiously or sarcastically.
"She's a gay woman. She'll drink anything and sleep with anyone."
Finally, as we peer with more and more intensity into the murky waters of history, we'll discover that up until the 1920's, the word "gay" meant "showing a merry, lively mood," etc, or something like it.
The word's etymology is said to be purely Germanic (Latin or Greek strains); but it did sneak into English by way of Old French. In Old Germanic it meant "fast, sudden." According to dictionary.com, the approximate time of its arrival (on Chaucer's estimable desk, no doubt) is the turn of the 13th Century:
The British Dictionary definition (the same link) mentions glibly that it wasn't any ordinary Old French that adopted the word from some unspecified Germanic dialect: it was Old Provençal, in which it was pronounced as "gai" (gahy).
I know not a word of Old French, nor indeed Old Provençal; but I do seem to recall that "gai" means "cheerful" in today's French.
Now. In their understandably hasty zest, the previous answerers seem to have overlooked three words that are at least as good as "merry," and probably better; those are:
which comes pretty damn close (but no cigar); and which Dictionary.com defines as, and I quote:
in good spirits; gay; merry:
which would be a wonderful choice if anyone ever used it except well-read people; and
which, unfortunately, cannot be sung with any confidence. Here's why. You'll recall the classic song from the classic movie whose title escapes me just now: "I'm going to gay Paree" in quick staccato mode; now try singing "I'm going to blithe Paree" just as quickly and see if it comes out intelligibly.
That's the scoop, so far. I hope someone sees it as a good challenge (which it is), remembers that the language he or she speaks is the language of Shakespeare, Milton and the Bible, as the poet said, and comes up with a better answer.