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.
A contemptible constable; an incompetent:Apparently sometimes used jovially to mean simply ''cop'': I don't believe that hump! I'm trying to develop a case, and he acts like I'm a hysterical old maid looking for rapists under the bed (1990+ Police)