Green’s Dictionary of Slang

1. the vagina; thus churn v. to move the body during sexual intercourse.

1787		Harris’s List of Covent-Garden Ladies 53: [She] is said to have not only a delicate hand at stroking, but great skill in the use of the churn, soon making love’s butter from nature’s cream.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 10: Baratte = churn = female pudendum.
c.1952		W. Harris [song title] Keep on churnin’ till the butter comes, / Keep on pumpin’, make the butter flow / Wipe off the paddle and churn some more.

2. (US) in pl., a pair of boots.

1891

Emporia Dly Repub. (KS) 28 Apr. 3/2: Trousers are ‘kicks,’ shoes are ‘squills’ or ‘stamps,’ and boots are ‘churns’.

3. (US) a teenage girl.

1949

Monteleone Criminal Sl. (rev. edn) 51: churns Young girls of teen-age.