Green’s Dictionary of Slang

cheeks n.1

[there is a single citation in 1660, the term is then lost (at least from print) until Jon Bee in 1823 and only resurfaces again in James Joyce’s Ulysses (1922); however, the phrase blind cheeks under blind adj.1 offers citations meaning buttocks for the whole of the 17C, 18C and 19C; also note 18C citations at ask cheeks near Cunnyborough ]

the posterior, the buttocks.

implied in blind cheeks under blind adj.1
[UK]Wandring Whore II 13: A third laughing at the large pair of cheeks and haunches she hath got.
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
[Ire]Joyce Ulysses 91: Shift stuck between the cheeks behind.
[US](con. WWII) J.O. Killens And Then We Heard The Thunder (1964) 201: All right soldier, bend over and spread your cheeks. [Ibid.] 356: She wet a cheek of his bashful backside and let him have the needle tenderly.
[US]F. Salas Tattoo the Wicked Cross (1981) 15: Bend over and spread your cheeks.
[US]J. Wambaugh Choirboys (1976) 63: Bend over and spread your cheeks.
[US]H. Max Gay (S)language.
[US]R.O. Scott Gay Sl. Dict. 🌐.
[US]S. King Dolores Claiborne 9: No high-steppin kitty like Vera Kiss-My-Back-Cheeks Donovan.
[Ire]J. O’Connor Salesman 276: Together again, Homer. Like the cheeks of me arse.
[UK]L. Gould Shagadelically Speaking 60: heinie, butt; bum; [...] seat; cheeks.

In phrases

ask cheeks near Cunnyborough

a woman-only phr., equivalent to ask my...! excl.

[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Cheeks, ask cheeks near cunnyborough, the repartee of a St. Giles’s fair one, who bids you ask her backside, anglice her a—se.
[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: [added to 1785 def.] A like pun is current in France. Any one asking the Road & Distance to Macon a City near Lyons would be answered by a French Lady of easy virtue Mettez votre nez dans mon cul & vous serrez dans les Faubourgs .
[UK]Lex. Balatronicum.
beat cheeks (v.) [var. on beat ass v.]

(US) to leave (at speed).

[US]S. Morgan Homeboy 115: C.C. asked what she was going to do. ‘Beat cheeks down the blacktop.’.