"male genital exhibitionist," 1960s (though meat-flasher in this sense was attested in 1890s); agent noun from flash (v.).
late 14c., from flasken (c.1300) "to dash or splash" (as water), probably imitative. Related: Flashed; flashing. Sense of "give off a sudden burst of light or flame" is 1540s. Flash flood is from 1940. Flash card is from 1923. Flash cube (remember those?) is from 1965.
1560s, from flash (v.); originally of lightning. Meaning "first news report" is from 1857. Meaning "photographic lamp" is from 1913. The comic book character dates to 1940. Flash in the pan (1809) is from old-style guns, where the powder might ignite in the pan but fail to spark the main charge.
A person who exposes the genitals in public; exhibitionist
[1960s+;maleexhibitionistswereknown as meat-flashers by the early 1890s]