Definition of "buttock"

buttock

/ˈbʌtək/

noun
plural buttocks
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.
  Quotations
  Run, run, come you hither / Novv, take all my Cuſhions dovvn and thvvack them ſoundly, / After my Feaſt of Millers: for their Buttocks / Have left a peck of flovver in them, beat them carefully […]
  c. 1615–1620 (date written), Tho[mas] Middleton, The Mayor of Quinborough: A Comedy. […], London: […] Henry Herringman, […], published 1661, Act V, scene i, page 61
2. (nautical) The convexity of a ship behind, under the stern.
  Quotations
  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