flasher n.

Middle English flashen, flasken (c. 1200), "sprinkle or splash (water, powder, etc.); to gush forth;" probably at least partly imitative (compare splash, dash), or in part from PIE *bhleu- "to swell, well up," extended form of root *bhel- (2) "to blow, swell." From c. 1400, of birds, "to dart or flit" also, of fire, "burst into flames." Some of the extended senses perhaps are from Scandinavian. Meanings "burst suddenly into view" (intransitive) and "emit or send forth suddenly" (transitive) are from 1580s. the Sense of "expose the genitals" is recorded by 1846. Related: Flashed; flashing. Flash card is from 1923.

1791, "act of creating an artificial flood," verbal noun from flash (v.); also compare flash (n.2)). Meaning "indecent exposure" is by 1968 (see flasher). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but the sense connection is unclear and it might be an unrelated word.