1680s (in hump-backed), from Dutch homp "lump," from Middle Low German hump "bump," from Proto-Germanic *hump-, from PIE *kemb- "to bend, turn, change, exchange." Replaced, or perhaps influenced by, crump, from Old English crump. A meaning attested from 1901 is "mound in a railway yard over which cars must be pushed," which may be behind the figurative sense of "critical point of an undertaking" (1914). Humpback whale is from 1725.
"to do the sex act with," attested from 1785, but the source of this indicates it is an older word. Meaning "to raise into a hump" is from 1840. Related: Humped; humping.
Most of the way to success or completion; over the hill
[1925+; a 1914 source defines hump as ''the half-way point in a prison sentence'']
[one of many fanciful coinages for something unspecified; probably related to hooter, ''anything trifling,'' found fr the mid-1800s, and to hewgag, ''an indeterminate, unknown mythical creature,'' similarly found; the syllable hoo-, which is prominent in such coinages, probably represents the interrogative pronoun who; the folk-music sense is based on this, in spite of a fanciful explanation by the singer Woody Guthrie, involving a loud singer called Hootin' Annie]