Green’s Dictionary of Slang

1. (US gay) an anal virgin.

1972

B. Rodgers Queens’ Vernacular 21: anal virgin [...] muffin (’40s–early ’50s).

2. (US) a young woman, a sweetheart.

1945		R.L. Bellem ‘Coffin for a Coward’ in Hollywood Detective Dec. 🌐 She was [...] a taffy-haired muffin whose gorgeous exterior concealed a heart as big as a birdseed.
1953		S. Fuller Pickup On South Street [film script] That muffin you grifted — she’s ok. Stuck her chin way out for you.
1975		L. Dills CB Slanguage.

3. (US gay) an attractive youth.

1965		Guild Dict. Homosexual Terms 31: muffin (n.): A young, attractive boy, as seen by an aroused homosexual.
2003		K. Cage Gayle 83/2: muffin n. gay man • stud muffin handsome gay man.

4. the vagina, or meton. as sexual intercourse.

1969		F. Norman in Fashion July in Norman’s London (1969) 251: Clap-ridden whores in Lisle Street [...] hawking gonorrhoea and their muffins — two pounds for a short time up against the wall.
1973		G.V. Higgins Digger’s Game (1981) 23: There ain’t no calories in muffin.
1987		‘Victoria Parker’ Pay for Play Cheerleaders 🌐 Finally one hand slipped under the jeans skirt, pushed forward and rubbed the hot little muffin between her legs.
1999		J. Cameron Brown Bread in Wengen [ebook] ‘Men you know they do always like a bit of muffin on the side’.

5. (US) in pl. the buttocks.

2013

T. Robinson Hard Bounce [ebook] As I stared at her exiting skirt muffins, I realized I’d just been had.