Green’s Dictionary of Slang

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

1965		(con. WWII) S. Linakis In Spring the War Ended 19: I mean, can you see me muffin’ some redheaded, sonofabitchin’ broad?
1972		B. Rodgers Queens’ Vernacular 139: muff [...] to tongue the clitoris and vulva.
1977		T. Berger Who is Teddy Villanova? 234: Whatinhell they really do, Rome? Muff one another?
2001		‘Penis Power’ ‘Masturbating Man’ on Camilla’s Erotic Stories Jun. 🌐 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)].

1997

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