Green’s Dictionary of Slang

Frenching n.

fellatio.

c.1915		in P. Smith Letter from My Father (1978) 56: Houses where they made a speciality of frenching and the houses were they staged shows, daisy-chains, etc.
c.1930		(ref. late 19C) N. Kimball Amer. Madam (1981) 89: These items of sexual life had various names over the years. [...] The act was most popularly known as Frenching.
c.1935		‘Peepin’ Through the Keyhole’ [comic strip] in B. Adelman Tijuana Bibles (1997) 52: What a Frenching she gave him.