'Know-it-all' definitions:

Definition of 'know-it-all'

From: WordNet
noun
Someone who thinks he knows everything and refuses to accept advice or information from others [syn: know-it- all, know-all]

Definition of 'Know-it-all'

From: GCIDE
  • 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]

Synonyms of 'know-it-all'

From: Moby Thesaurus