Green’s Dictionary of Slang

duck v.3

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

[US]G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
[US]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.
[US]B. Rodgers Queens’ Vernacular.