1920s euphemism for sex appeal, popularized by British writer Elinor Glyn in her 1926 novel of the same name (Rudyard Kipling used the word in print as early as 1904). "It is that quality possessed by some that draws all others with its magnetic force. "It" can be a quality of mind as well as a physical attraction (...) You either have it or you don't", quoth Glyn. When asked who possesses It in Hollywood, Glyn replied the only ones are actors Clara Bow and Antonio Moreno and the doorman at the Ambassador Hotel. Bow and Moreno were immediately cast for the 1927 film version, which transformed Bow into what Glyn called the "it" girl, and incidentally one of the biggest icons of the late 1920s and the epitome of flapperness.

The Marx Brothers's take on It (from 1931's Monkey Business):

Chico: Hey, you're a nice lookin'a girl, alright. You got 'it'!
Manicurist: Thank you.
Chico: And you can keep 'it'!