Green’s Dictionary of Slang

cocksuck n.

[backform. f. cocksucker n.]
(orig. US)

1. the act of fellatio.

[US] cited in Lighter HDAS I.
[US]Sandmann ‘Burning Down the House’ Planet Sex Stories [Internet] If you haven’t guessed it by now, I can tell you that I’m a real cocksuck queen. It’s not just that I have great technique, which I do, but I also just love slurping on a dick.

2. a general term of abuse.

[US](con. 1969) M. Herr Dispatches 138: Hey, cocksuck, you just tol’ me to turn it off.
[US]J. Ellroy Brown’s Requiem 73: Freshly painted obscenities covered the back wall near the workbench: ‘fuck’, ‘cocksuck’, ‘kill fuck,’ and ‘cocksuck bastard’.