1550s, "to turn up one's nose, sneer at;" later "to rap or flick, as with the fingers" (1560s); "throw with a sudden movement," also "move in short, quick flights" (1580s). Perhaps imitative (compare flip (v.), also East Frisian flirt "a flick or light blow," flirtje "a giddy girl," which also might have fed into the English word), but perhaps rather from or influenced by flit (v.). Related: Flirted; flirting.
The main modern verbal sense of "play at courtship" (1777) probably developed from the noun (see flirt (n.)) but also could have grown naturally from the 16c. meaning "to flit inconstantly from object to object." To flirt a fan (1660s) was to snap it open or closed with a brisk jerk and was long considered part of the coquette's arsenal, which might have contributed to the sense shift. Or the word could have been influenced from French, where Old French fleureter meant "talk sweet nonsense," also "to touch a thing in passing," diminutive of fleur "flower" (n.) and metaphoric of bees skimming from flower to flower. French flirter "to flirt" is a 19c. borrowing from English.
flirt (n.)
1540s, "joke, jest, stroke of wit, contemptuous remark," from flirt (v.). By 1560s as "a pert young hussey" [Johnson], and Shakespeare has flirt-gill (i.e. Jill) "a woman of light or loose behavior" (Fletcher formalizes it as flirt-gillian), while flirtgig was a 17c. Yorkshire dialect word for "a giddy, flighty girl." One of the many fl- words suggesting loose, flapping motion and connecting the notions of flightiness and licentiousness. Compare English dialect and Scottish flisk "to fly about nimbly, skip, caper" (1590s); source of Scott's fliskmahoy "girl giddy and full of herself." The meaning "person who plays at courtship" is from 1732 (as the name of female characters in plays at least since 1689 (Aphra Behn's "The Widow Ranter")). Also in early use sometimes "person one flirts with," though by 1862 this was being called a flirtee.