Green’s Dictionary of Slang

cocksuck v.

[backform. f. cocksucker n.]

(orig. US) to fellate.

[UK]J. Carr Bad (1995) 44: What kinda work you in, boy? Cocksuckin’ or cornholin’?
R. Hawkins Bimbos by the Bay (1998) 105: She cock-sucked him like crazy, and then he lost all control.
[US]‘Bill E. Goodhead’ Nubile Treat [Internet] He just couldn’t imagine how anything could possibly feel better than those lips of his mother’s on his tool. He was thrilled by the idea of being cock-sucked by his own Mom.