The Collaborative International Dictionary of English v.0.48:

mess up \mess up\, v. t.
   1. To make a mess[5] of; to confuse, disorder, or muddle; to
      muss; to jumble; to disturb.
      [PJC]

   2. To botch[3]; to bungle.
      [PJC]

WordNet (r) 3.0 (2006):

mess up
    v 1: make a mess of, destroy or ruin; "I botched the dinner and
         we had to eat out"; "the pianist screwed up the difficult
         passage in the second movement" [syn: botch, bodge,
         bumble, fumble, botch up, muff, blow, flub,
         screw up, ball up, spoil, muck up, bungle,
         fluff, bollix, bollix up, bollocks, bollocks up,
         bobble, mishandle, louse up, foul up, mess up,
         fuck up]
    2: disturb the smoothness of; "ruffle the surface of the water"
       [syn: ruffle, ruffle up, rumple, mess up]
    3: make a mess of or create disorder in; "He messed up his room"
       [syn: mess, mess up]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "mess up":
   ball up, befoul, benasty, bitch, bitch up, blur, bobble, bollix,
   bollix up, bonehead into it, bugger, bugger up, clutter,
   complicate, confound, confuse, contaminate, cook, corrupt, damage,
   defile, deform, derange, disarrange, disarray, discompose,
   dishevel, dislocate, disorder, disorganize, distort, disturb,
   drop a brick, embrangle, entangle, flub, fog up, foul, foul up,
   goof, goof up, gum up, hash up, implicate, infect, involve, jumble,
   knot, litter, louse up, mar, mess, misarrange, mix up, muck,
   muck up, mucker, muddle, muff, muss, muss up, nasty, obfuscate,
   obscure, perplex, play hell with, play hob with, pollute, queer,
   ramify, ravel, ruffle, ruin, rummage, rumple, scatter, screw up,
   sink, snafu, snarl, snarl up, spoil, sully, taint, tangle, tarnish,
   tousle, unform, unsettle, unshape, upset