The Collaborative International Dictionary of English v.0.48:

guff \guff\ n.
   1. A ludicrously false statment. [WordNet sense 1]

   Syn: bullshit, bull, Irish bull, horseshit, shit, crap, bunk,
        bunkum, buncombe, rot, hogwash, dogshit.
        [WordNet 1.5]

   2. Insolent or disrespectful talk; as, don't give me any of
      your guff, young lady.
      [PJC]

WordNet (r) 3.0 (2006):

guff
    n 1: unacceptable behavior (especially ludicrously false
         statements) [syn: bunk, bunkum, buncombe, guff,
         rot, hogwash]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "guff":
   babble, babblement, balderdash, baloney, bavardage, bibble-babble,
   bilge, blab, blabber, blah, blah-blah, blather, blether, blethers,
   bop, bosh, bull, bullshit, bunk, bunkum, cackle, caquet,
   caqueterie, chat, chatter, chitter-chatter, clack, claptrap,
   clatter, crap, flapdoodle, gab, gabble, gas, gibber, gibble-gabble,
   gossip, gup, hogwash, hokum, hooey, hot air, idle talk, jabber,
   lip, malarkey, mere talk, moonshine, mouth, natter, nonsense talk,
   palaver, piffle, poppycock, prate, prating, prattle,
   prittle-prattle, rot, sass, sauce, scat, shit, talkee-talkee,
   tittle-tattle, tommyrot, trash, tripe, twaddle, twattle, wind, yak,
   yakkety-yak