An Etymological Dictionary of Astronomy and Astrophysics

The hind part of the body of an animal, as the hindquarters of a quadruped or sacral region of a bird (Dictionary.com).

Etymology (EN): M.E. rumpe, from Scandinavian; compare Dan., Norw. Swed. rumpe rumpa “tail;” cognate with Ger. Rumpf “body, trunk.”

Etymology (PE): Kapal, maybe from Ar. kafal.
Sorin, → buttock.