"sex with a woman; woman regarded as a sex object; female genitalia," c. 1910, of uncertain origin, probably via New Orleans Creole, from French putain "prostitute," from Old French pute "whore" (cognate with Spanish and Provençal puta), probably from fem. of Vulgar Latin *puttus "girl" (source of Old Italian putta "girl"), from Latin putus (originally "pure, bright, splendid").

But also possibly from or influenced by Old French put, from Latin putidus "stinking" on notion of the "foulness" of harlotry [Buck], or for more literal reasons (among the 16c.-17c. slang terms for "whore" in English were polecat, which might also be a pun, and fling-stink). Shortened form poon is recorded from 1969.