buttock

Etymology

Inherited from Middle English buttok, probably from Old English buttuc (“end; end piece”; also, “short piece of land”). Attested with its current anatomical meaning since 1300. A diminutive form of what is presumably the Old English precursor of butt + -ock (diminutive suffix).

noun

1. (usually in the plural) Each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs.
2. (nautical) The convexity of a ship behind, under the stern.
   There came a blast of freezing wind that made Skell shrug himself against the oaken post on which the ship's buttock rested. 1925, Adventure, volume 54