The Collaborative International Dictionary of English v.0.48:

Mar \Mar\, v. t. [imp. & p. p. Marred (m[aum]rd); p. pr. & vb.
   n. Marring.] [OE. marren, merren, AS. merran, myrran (in
   comp.), to obstruct, impede, dissipate; akin to OS. merrian,
   OHG. marrjan, merran; cf. D. marren, meeren, to moor a ship,
   Icel. merja to bruise, crush, and Goth. marzjan to offend.
   Cf. Moor, v.]
   1. To make defective; to do injury to, esp. by cutting off or
      defacing a part; to impair; to disfigure; to deface.
      [1913 Webster]

            I pray you mar no more trees with wiring love songs
            in their barks.                       --Shak.
      [1913 Webster]

            But mirth is marred, and the good cheer is lost.
                                                  --Dryden.
      [1913 Webster]

            Ire, envy, and despair
            Which marred all his borrowed visage. --Milton.
      [1913 Webster]

   2. To spoil; to ruin. "It makes us, or it mars us." "Striving
      to mend, to mar the subject." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

marred
    adj 1: blemished by injury or rough wear; "the scarred piano
           bench"; "walls marred by graffiti" [syn: marred,
           scarred]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "marred":
   bandy, bandy-legged, battered, beautiless, blemished, blighted,
   bloated, blotted, botched, bowlegged, bruised, cacophonic,
   cacophonous, checked, cicatrized, club-footed, cracked, crazed,
   defaced, defective, deformed, destroyed, disfigured, distorted,
   dwarfed, dysphemistic, dysphemized, faulty, flatfooted, flawed,
   grotesque, homely, ill-made, ill-proportioned, ill-shaped,
   impaired, inelegant, keloidal, kinked, knock-kneed, malformed,
   misbegotten, misproportioned, misshapen, monstrous, mutilated,
   out of shape, pigeon-toed, pimpled, pimply, plain, pug-nosed,
   rachitic, rickety, ruined, scabbed, scabby, scarified, scarred,
   short on looks, simous, snub-nosed, split, spoiled, spoilt, stumpy,
   swaybacked, talipedic, truncated, twisted, uglified, ugly,
   ugly as hell, ugly as sin, unaesthetic, unattractive, unbeautiful,
   uncomely, unhandsome, unlovely, unpleasing, unpretty, unsightly,
   warped, wrecked