Yours, Etc.: Origins and Uses of 8 Sign-Offs
1823, American English college slang, original meaning "to back out, give up, fail," traditionally said to be an alteration of British university slang funk "to be frightened, shrink from" (see funk (n.1)). Related: Flunked; flunking.
: I've got three passes and two flunksverb
[1823+ College; origin unknown; perhaps a blend of fail with funk, perhaps echoic of a dull collapse]