Green’s Dictionary of Slang

1. to engage in lesbian sex, spec. the rubbing together of genitals;thus dyking, lesbian intercourse.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants II 163: [D]iking, is often used to refer to Lesbian intercourse, especially tribady; and also, infrequently, to the intercourse of two homosexual men with each other. [...] [A]s a verb, dike is seldom used alone, being more often combined with a preposition, as dike with or diking together.
1965		Guild Dict. Homosexual Terms 11: diking (it); used to refer to lesbian intercourse.
1972		B. Rodgers Queens’ Vernacular.
1993		(con. 1950s) Kennedy & Davies Boots of Leather (2014) 227: ‘What is dyking? It’s when a butch and a fem, the fem plays the woman part and the butch plays the male part, and the male lays on the female just like a man would do to a woman, except for there’s no intercourse [...] you don’t feel the penis inside you’.

2. to live as a lesbian.

1963		Kerouac letter 14 Jan. in Charters II (1999) 356: Here he is writhing in torment in Connecticutt-I-guess beds as she dikes ahead in Paree.
1968		G. Vidal Myra Breckinridge 272: [She] gave up dyking on the spot to become a Jehovah’s witness.