The Collaborative International Dictionary of English v.0.48:

Grotesque \Gro*tesque"\ (gr[-o]*t[e^]sk"), a. [F., fr. It.
   grottesco, fr. grotta grotto. See Grotto.]
   1. Like the figures found in ancient grottoes; grottolike.
      [1913 Webster]

   2. Hence: Wildly or strangely formed; whimsical; extravagant;
      of irregular forms and proportions; fantastic; ludicrous;
      antic. "Grotesque design." --Dryden. "Grotesque
      incidents." --Macaulay.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grotesque \Gro*tesque\, n.
   1. A whimsical figure, or scene, such as is found in old
      crypts and grottoes. --Dryden.
      [1913 Webster]

   2. Artificial grotto-work.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grotesque
    adj 1: distorted and unnatural in shape or size; abnormal and
           hideous; "tales of grotesque serpents eight fathoms long
           that churned the seas"; "twisted into monstrous shapes"
           [syn: grotesque, monstrous]
    2: ludicrously odd; "Hamlet's assumed antic disposition";
       "fantastic Halloween costumes"; "a grotesque reflection in
       the mirror" [syn: antic, fantastic, fantastical,
       grotesque]
    n 1: art characterized by an incongruous mixture of parts of
         humans and animals interwoven with plants

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "grotesque":
   Gothic, aberrant, abnormal, absurd, anomalous, antic, art object,
   bandy, bandy-legged, baroque, beyond belief, bizarre, blemished,
   bloated, bowlegged, brain-born, brainchild, bric-a-brac, cacogenic,
   classic, club-footed, cockamamie, comic, comical, composition,
   crazy, creation, curious, defaced, deformed, design, disfigured,
   distorted, dream-built, droll, dwarfed, eerie, extravagant,
   extreme, fanciful, fancy-born, fancy-built, fancy-woven, fantasque,
   fantastic, flamboyant, flatfooted, florid, foolish, freak,
   freakish, gargoylish, gnarled, gruesome, high-flown, ill-made,
   ill-proportioned, ill-shaped, incongruous, incredible, kitsch,
   knock-kneed, laughable, ludicrous, maggoty, malformed, marred,
   master, masterpiece, masterwork, misbegotten, misproportioned,
   misshapen, mobile, monstrous, museum piece, mutilated, nonsensical,
   notional, nude, odd, offbeat, old master, out of shape, outlandish,
   outrageous, outre, pasticcio, pastiche, peculiar, piece,
   piece of virtu, pigeon-toed, poppycockish, preposterous, pug-nosed,
   queer, rachitic, rickety, ridiculous, rococo, shapeless, simous,
   snub-nosed, stabile, statue, still life, strange, study, stumpy,
   swaybacked, talipedic, teratic, teratogenic, teratoid, truncated,
   twisted, uncanny, unshapely, virtu, weird, whimsical, wild, work,
   work of art