cheek (n.) Look up cheek at Dictionary.com
Old English ceace, cece "jaw, jawbone," in late Old English also "the fleshy wall of the mouth." Perhaps from the root of Old English ceowan "chew" (see chew (v.)), or from Proto-Germanic *kaukon (cognates: Middle Low German kake "jaw, jawbone," Middle Dutch kake "jaw," Dutch kaak), not found outside West Germanic.

Words for "cheek," "jaw," and "chin" tend to run together in IE languages (compare PIE *genw-, source of Greek genus "jaw, cheek," geneion "chin," and English chin); Aristotle considered the chin as the front of the "jaws" and the cheeks as the back of them. The other Old English word for "cheek" was ceafl (see jowl).
A thousand men he [Samson] slow eek with his hond,
And had no wepen but an asses cheek.
[Chaucer, "Monk's Tale"]
In reference to the buttocks from c.1600. Sense of "insolence" is from 1840, perhaps from a notion akin to that which led to jaw "insolent speech," mouth off, etc. To turn the other cheek is an allusion to Matt. v:39 and Luke vi:29.