Etymology
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flak (n.)
1938, "anti-aircraft gun," from German Flak, condensed from Fliegerabwehrkanone, literally "pilot warding-off cannon." Sense of "anti-aircraft fire" is from 1940; metaphoric sense of "criticism" is c. 1963 in American English. Flak jacket is by 1956.
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flack (n.)

"publicity or press agent," 1945, also by that year as a verb, said to have been coined at show biz magazine Variety (but the first attested use is not in Variety) and supposedly from name of Gene Flack, a movie agent, but influenced by flak. There was a Gene Flack who was an advertising executive in the U.S. during the 1940s, but he seems to have sold principally biscuits, not movies.

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flake (n.)

"thin flat piece of snow; a particle," early 14c., also flauke, flagge, which is of uncertain origin, possibly from Old English *flacca "flakes of snow," or from cognate Old Norse flak "flat piece," from Proto-Germanic *flakaz (source also of Middle Dutch vlac, Dutch vlak "flat, level," Middle High German vlach, German Flocke "flake"); from PIE root *plak- (1) "to be flat." From late 14c. as "a speck, a spot."

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fluke (n.3)

"flatfish," Old English floc "flatfish," related to Old Norse floke "flatfish," flak "disk, floe," from Proto-Germanic *flok-, from PIE root *plak- (1) "to be flat." The parasite worm (1660s) so called from resemblance of shape.

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-happy 
word-forming element used in World War II armed forces slang and after, meaning "crazed or frazzled from stress due to" the thing specified (as in bomb-happy (1942), flak-happy (1943), trigger-happy (1942). The model might have been slap-happy in pugilism from 1936 as a slang variant of "punch-drunk."
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corn flakes (n.)

also corn-flakes, cornflakes, type of breakfast cereal, 1902, from corn (n.1) + flake (n.); the process of making them was discovered by American food manufacturer W.K. Kellogg in 1894. Earlier corn flakes was an ingredient in brewing (1890s).

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flaky (adj.)

1570s, "consisting of flakes," from flake + -y (2). Meaning "eccentric, crazy" first recorded 1959, said to be American English baseball slang, but probably from earlier druggie slang flake "cocaine" (1920s). Flake (n.) "eccentric person" is a 1968 back-formation from it. Related: Flakiness.

The term 'flake' needs explanation. It's an insider's word, used throughout baseball, usually as an adjective; someone is considered 'flaky.' It does not mean anything so crude as 'crazy,' but it's well beyond 'screwball' and far off to the side of 'eccentric.' [New York Times, April 26, 1964]
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flake (v.)
early 15c., flaken, (of snow) "to fall in flakes," from flake (n.). Transitive meaning "break or peel off in flakes" is from 1620s; intransitive sense of "to come off in flakes" is from 1759. . Related: Flaked; flaking.
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