"showy, cheaply attractive," 1680s, from flash (n.1) + -y (2). Earlier it meant "splashing" (1580s); "sparkling, giving off flashes" (c. 1600), but those senses have become rare. Related: Flashily; flashiness.
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."
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
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.
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Definitions of flashy from WordNet
(used especially of clothes) marked by conspicuous display;