The Collaborative International Dictionary of English v.0.48:

Eyesore \Eye"sore`\, n.
   Something offensive to the eye or sight; a blemish.
   [1913 Webster]

         Mordecai was an eyesore to Haman.        --L'Estrange.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eyesore
    n 1: something very ugly and offensive

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "eyesore":
   baboon, bag, blemish, bloodstain, blot, blotch, blur, brand, dab,
   daub, dog, fleck, flick, flyspeck, fright, gargoyle, hag, harridan,
   macula, maculation, macule, mark, mess, monster, monstrosity,
   no beauty, patch, scarecrow, sight, smear, smirch, smouch, smudge,
   smut, smutch, spatter, speck, speckle, splash, splatter, splotch,
   spot, stain, stigma, taint, tarnish, teratism, ugly duckling,
   witch