The English word buttocks presents a problem for speakers: in the standard language, it’s both formally plural (with final /s/) and grammatically plural (rejects singular determiners like a, takes plural verb agreement: *a buttocks, Kim’s buttocks were/*was perfectly symmetrical). However, buttocks plays a double role semantically:
it denotes a matched pair of body-parts (NOAD2′s definition for buttock: “either of the two round fleshy parts that form the lower rear area of a human trunk”; buttocks gets only a subentry here: “the rump of an animal”); this is buttock, roughly ‘ass-cheek’, seen in left buttock vs. right buttock
and it denotes the two as a unit — roughly, ‘ass, butt, bottom, behind, backside, rear end, …’ (for which I’ll use the gloss ‘butt’) — in which case it acts somewhat like a plurale tantum (cf. pants), plural formally and grammatically, but singular in reference, and somewhat like formally plural singular nouns (shingles the disease, linguistics, etc. — there are several subtypes of these, well studied)
How to reconcile this double role?