Green’s Dictionary of Slang

to bend over in preparation for anal intercourse; usu. in phr. fuck, suck and duck.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 13: duck (v.): To allow pedication; it is never used alone, but in conjunction with the phrase: ‘To fuck, suck and duck,’ since it can mean to bend forward suddenly.
1972		B. Rodgers Queens’ Vernacular.