The term "buttocks" refers to the two rounded cheeks at the backside or rear end of a person. It is also commonly used as slang for someone's behind, rump or derriere.
Hence the presence of buttocks cut from porn magazines, equivalents to the putti in Renaissance depictions of the Virgin. (artspace.com)
The sciatic nerve runs very close to this muscle and tension in the muscle can cause compression of the sciatic nerve resulting in buttock pain which radiates and down the leg. (sportsinjuryclinic.net)
The giant Recaro seats initially feel firm on buttock and back, but you soon get the sense they've been ergonomically contoured. (evo.co.uk)