The Collaborative International Dictionary of English v.0.48:

Know-it-all \Know"-it-all`\, n.
   One who knows everything; hence, a person who makes
   pretension to great knowledge, especially one whose didactic
   conversational habit conspicuously reveals his belief that he
   has superior knowledge on many subjects; a wiseacre; a
   know-all; -- usually ironical. [Colloq. & pejorative]

   Note: the use of this term implies that the speaker
         disapproves of this behavior, and may think that it is
         unjustified
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

know-it-all
    n 1: someone who thinks he knows everything and refuses to
         accept advice or information from others [syn: know-it-
         all, know-all]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "know-it-all":
   Narcissus, aggressively self-confident, biggety, braggart,
   bumptious, chesty, cocky, conceited, egocentric, egoist, egotist,
   immodest, individualist, narcissist, narcist, no modest violet,
   obtrusive, overwise, peacockish, peacocky, perk, perky, pert,
   puffed up, self-conceited, self-opinionated, smart aleck,
   smart-alecky, smarty, smarty-pants, stuck-up, swelled-headed,
   swellhead, wise guy, wiseacre, wisecracker, wisehead,
   wisenheimer