"hind-quarters, back-end, or buttocks of an animal," the part to which the tail is attached, mid-15c., from a Scandinavian source (compare Danish, Norwegian rumpe, Swedish rumpa), from or corresponding to Middle Dutch romp, German Rumpf "trunk, torso."
Specifically of this part used as food by late 15c. The figurative sense of "small remnant, tail-end of anything" derives from the notion of "tail" and is recorded from 1640s in reference to the English Rump Parliament (December 1648-April 1653), so called from sitting after the expulsion of the majority of members of the Long Parliament in Pride's Purge. As an adjective from c. 1600. Also in 18c. and 19c. a verb, "to turn one's back to" as a snub.