The Collaborative International Dictionary of English v.0.48:

Wiseacre \Wise"a*cre\, n. [OD. wijssegger or G. weissager a
   foreteller, prophet, from weissagen to foretell, to prophesy,
   OHG. w[imac]ssag?n, corrupted (as if compounded of the words
   for wise and say) fr. w[imac]zzag?n, fr. w[imac]zzag? a
   prophet, akin to AS. w[imac]tiga, w[imac]tga, from the root
   of E. wit. See Wit, v.]
   [1913 Webster]
   1. A learned or wise man. [Obs.]
      [1913 Webster]

            Pythagoras learned much . . . becoming a mighty
            wiseacre.                             --Leland.
      [1913 Webster]

   2. One who makes undue pretensions to wisdom; a would-be-wise
      person; hence, in contempt, a simpleton; a dunce.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wiseacre
    n 1: an upstart who makes conceited, sardonic, insolent comments
         [syn: wise guy, smart aleck, wiseacre, wisenheimer,
         weisenheimer]