hump (n.) Look up hump at
1680s (in hump-backed), of uncertain origin; perhaps from Dutch homp "lump," from Middle Low German hump "bump," from Proto-Germanic *hump-, from PIE *kemb- "to bend, turn, change, exchange" (see change (v.)). 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 might be behind the figurative sense of "critical point of an undertaking" (1914).
hump (v.) Look up hump at
"to bend or raise into a hump," 1840, from hump (n.). Meaning "do the sex act with" is attested from 1785, but the source indicates it is an older word. Related: Humped; humper; humping.