Green’s Dictionary of Slang

(gay) to masturbate.

1916		H.N. Carey Sl. Venery I 44: churning—Masturbation.
1959		‘A. Edwardes’ Jewel in Lotus 113: [no text] [Simes:DLSS].
1986		W. Leyland Hot Studs 92: [T]he rest of the guys waited around for their turn, churning their meat [Simes:DLSS].