hump (v.) Look up hump at Dictionary.com
"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.
hump (n.) Look up hump at Dictionary.com
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).