Green’s Dictionary of Slang

muff v.2

1. (US) to perform cunnilingus [abbr. muff-dive v.].

(con. WWII) S. Linakis In Spring the War Ended 19: I mean, can you see me muffin’ some redheaded, sonofabitchin’ broad?
[US]B. Rodgers Queens’ Vernacular 139: muff [...] to tongue the clitoris and vulva.
[US]T. Berger Who is Teddy Villanova? 234: Whatinhell they really do, Rome? Muff one another?
‘Penis Power’ ‘Masturbating Man’ on Camilla’s Erotic Stories Jun. [Internet] I waited the weekends eagerly for her increasingly sophisticated blow jobs, and I much enjoyed muffing her and going down on and sucking and finger-fucking her seemingly near insatiate cunt.

2. to have sexual intercourse with [muff n.1 (1)].

[UK]Roger’s Profanisaurus in Viz 87 Dec. n.p.: muff 1. [...] 2. v. To shag; have intercourse with.