not a native letter in Old English; in Anglo-French words it represents the "ts" sound (as in Anglo-French fiz, from Latin filius, modern Fitz); from late 13c. it began to be used for the voiced "s" sound and had fully taken that role by 1400. For letter name, see zed.
Thou whoreson Zed, thou vnnecessary Letter. ["King Lear," II.ii.69]
Series of zs to represent a buzzing sound first attested 1852; zees "spell of sleep, a nap" is slang first recorded 1963, American English student slang.
"female genitalia," also "sex with a woman; woman regarded as a sex object," c. 1910, a word of uncertain origin. Shortened form poon is recorded by 1969.
Probably via New Orleans Creole, from French putain "prostitute," from Old French pute "whore" (cognate with Spanish and Provençal puta), probably from 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).
Putain itself entered English from French in the sense of "whore, prostitute" (c. 1300), mostly in the phrase fitz a putain "whoreson, son of a whore." Putage in old legal language meant "prostitution" (late 15c.).