There are, of course, several theories about the origin of the adjective “fly.” Perhaps the simplest ties the “clever, in the know, not easily fooled” sense to the common fly, always vigilant and almost impossible to catch off-guard. The fact that this slang “fly” originated in the underworld, where scams and ruses were the rule and to let one’s guard down could be fatal, gives this theory some plausibility. Another theory, offered by the eminent slang etymologist Eric Partridge, suggests that this “fly” is actually a form of “fledge,” which we use to mean “mature” (as in “full-fledged”), but originally referred to a young bird that had grown enough feathers to fly. Since “fledge” is rooted in “fly” itself, we’re still in the ballpark with the verb “to fly” with that theory. Another theory suggests that “fly” was originally “fla,” a short form of “flash,” used in the 18th and 19th century as slang meaning “clever, in the know” (from “flash” meaning “intense light,” in this case a metaphor for intelligence).

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1 comment on this post.
  1. Melvin Sim:

    Dear Word Detective,

    It’s coming up to a baker’s dozen of months since this column. Would you be able to follow up on your promise to tackle the history of the word “gay”?

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