Green’s Dictionary of Slang

(US) mutual oral-genital stimulation.

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, but it was hardly a French speciality. If mutually played, it was 69 or fork and spoon.