Flunk

google

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early 19th century (in the general sense ‘back down, fail utterly’; originally US): perhaps related to funk1 or to US flink ‘be a coward’, perhaps a variant of flinch.

wiktionary

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Alteration of funk, or perhaps a blend of flinch +‎ funk.

etymonline

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flunk (v.)

1823, American English college slang, original meaning "to back out, give up, fail," of obscure origin, traditionally said to be an alteration of British university slang funk "to be frightened, shrink from" (see funk (n.1)). Meaning "cause to fail, give a failing mark to" is from 1843. Related: Flunked; flunking.