buttock

Main Entry: but·tock 

Pronunciation: \ˈbə-tək also -(ˌ)täk\

Function: noun

Etymology: Middle English buttok — more at butt

Date: 14th century

1 : the back of a hip that forms one of the fleshy parts on which a person sits
2 plural a : the seat of the body b : rump 1a