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.
"sudden rush of water," 1660s, earlier "watery place or marsh, a swamp" (c. 1400; in place names from c. 1300), of uncertain origin or connection to flash (n.1); perhaps from Old French flache, from Middle Dutch vlacke. Flash flood is from 1940.
1560s, "sudden burst of flame or light," from flash (v.); originally of lightning. Figuratively (of wit, laughter, anger, etc.) from c. 1600. Meaning "period occupied by a flash, very short time" is from 1620s. Sense of "superficial brilliancy" is from 1670s. Meaning "first news report" is from 1857. The comic book character dates to 1940. Meaning "photographic lamp" is from 1913. Flash cube (remember those?) is from 1965.
Flash in the pan (1704 literal, 1705 figurative) is from old-style firearms, where the powder might ignite in the pan but fail to spark the main charge; hence figurative sense "brilliant outburst followed by failure."
"rapid attack," 1939, from German Blitzkrieg, from Krieg "war" (see kriegspiel) + Blitz "lightning," from Middle High German blicze, back-formation from bliczen "to flash," from Old High German blecchazzen "to flash, lighten" (8c.), from Proto-Germanic *blikkatjan, from PIE root *bhel- (1) "to shine, flash, burn."
early 15c., "publish a 'thundering' denunciation; hurl condemnation (at an offender)," a figurative use, from Latin fulminatus, past participle of fulminare "hurl lightning, lighten," figuratively "to thunder," from fulmen (genitive fulminis) "lightning flash," related to fulgor "lightning," fulgere "to shine, flash," from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn." Metaphoric sense (the oldest in English) in reference to formal condemnation is from Medieval Latin fulminare, used of formal ecclesiastical censures. Related: Fulminated; fulminating.
"state or character of shining brightly; a flood of light," 1630s, from Latin refulgentia "reflected luster, splendor," from refulgens, present participle of refulgere "flash back, shine brilliantly," from re- "back" (see re-) + fulgere "to shine" (from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn"). Related: Refulgency (1610s).
1590s, "unite or consolidate by hammering or compression, often after softening by heating," alteration of well (v.) "to boil, rise;" influenced by past participle form welled. Related: Welded; welding.