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