flash (v.) Look up flash at Dictionary.com
Middle English flashen, flasken (c.1200), "sprinkle or splash (water, powder, etc.); to gush forth;" probably at least partly imitative (compare splash, dash). 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.
flash (n.1) Look up flash at Dictionary.com
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."
flash (n.2) Look up flash at Dictionary.com
"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.
flash (adj.) Look up flash at Dictionary.com
from flash (v.) in various and unconnected senses, often slang; sense of "of or associated with thieves, prostitutes, etc." is from c.1700. That of "vulgar, showy" is from 1785 (it is older in flashy). That of "expert, smart" is from 1812.