Green’s Dictionary of Slang

1. to fellate.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1972		B. Rodgers Queens’ Vernacular.
2010		W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 64: When a man kneeled to do this [i.e. fellate an anonymous partner] , he was described as doing church or kneeling at the altar.

2. to sodomize.

1927		‘Jargon of the Und.’ in DN V 453: Kneel at the altar, To commit pederasty.
1931		Irwin Amer. Tramp and Und. Sl. 118: Kneeling at the Altar. – Committing pederasty.
1949		Monteleone Criminal Sl. (rev. edn).
1962		Ragen & Finston World’s Toughest Prison 806: kneeling at the altar – Committing pederasty.
1965		Guild Dict. Homosexual Terms 26: kneel at the altar (v.): The practice of pedication. (Slang.).
1989–2003		R.O. Scott Gay Sl. Dict. 🌐 anal intercourse: [...] Syn: kneeling at the altar.