An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Either of the two large fleshy masses of thick muscular tissue that form the human rump.
2) In animals, the → rump (Dictionary.com).

M.E. buttok, maybe from O.E. buttuc "end; end piece; short piece of land."

Sorin, variant sarun; Mid.Pers. sarinak, srinak (Sogdian šun / šôn "hip, haunch"); Av. sraoni- "buttok, hip;" Skt. śroni- "the hip and loins," Pali soni- "the buttock."