The Collaborative International Dictionary of English v.0.48:

cocksucker \cock"suck*er\ n.
   a worthless or despicable person; -- intended as disparaging
   and invariably offensive. [vulgar slang]

   Syn: rotter, rat, skunk, stinker, bum, crumb, lowlife, scum
        bag, motherfucker, mother.
        [WordNet 1.5]

   2. a person who performs fellatio.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

cocksucker
    n 1: a person who performs fellatio
    2: insulting terms of address for people who are stupid or
       irritating or ridiculous [syn: asshole, bastard,
       cocksucker, dickhead, shit, mother fucker,
       motherfucker, prick, whoreson, son of a bitch, SOB]