The Collaborative International Dictionary of English v.0.48:

motherfucker \moth"er*fuck`er\ n.
   1. A person who is deemed to be worthless or despicable; --
      obscene and highly offensive. [vulgar slang]

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

   2. Any object viewed as annoying or frustrating; -- usually
      used as an expression of anger. [vulgar slang]
      [PJC]

WordNet (r) 3.0 (2006):

motherfucker
    n 1: 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]