The Collaborative International Dictionary of English v.0.48:

Stun \Stun\, v. t. [imp. & p. p. Stunned; p. pr. & vb. n.
   Stunning.] [OE. stonien, stownien; either fr. AS. stunian
   to resound (cf. D. stenen to groan, G. st["o]hnen, Icel.
   stynja, Gr. ?, Skr. stan to thunder, and E. thunder), or from
   the same source as E. astonish. [root]168.]
   1. To make senseless or dizzy by violence; to render
      senseless by a blow, as on the head.
      [1913 Webster]

            One hung a poleax at his saddlebow,
            And one a heavy mace to stun the foe. --Dryden.
      [1913 Webster]

   2. To dull or deaden the sensibility of; to overcome;
      especially, to overpower one's sense of hearing.
      [1913 Webster]

            And stunned him with the music of the spheres.
                                                  --Pope.
      [1913 Webster]

   3. To astonish; to overpower; to bewilder.
      [1913 Webster]

            William was quite stunned at my discourse. --De Foe.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Stunning \Stun"ning\, a.
   1. Overpowering consciousness; overpowering the senses;
      especially, overpowering the sense of hearing; confounding
      with noise.
      [1913 Webster]

   2. Striking or overpowering with astonishment, especially on
      account of excellence; as, stunning poetry. [Slang] --C.
      Kingsley. -- Stun"ning*ly, adv. [Slang]
      [1913 Webster]

WordNet (r) 3.0 (2006):

stunning
    adj 1: commanding attention; "an arresting drawing of people
           turning into animals"; "a sensational concert--one never
           to be forgotten"; "a stunning performance" [syn:
           arresting, sensational, stunning]
    2: causing great astonishment and consternation; "the strike
       came as a stunning protest against management"; "a stunning
       defeat"
    3: causing or capable of causing bewilderment or shock or
       insensibility; "laid the poor fellow senseless with one
       stunning blow"; "a stunning detonation with volumes of black
       smoke"
    4: strikingly beautiful or attractive; "quite stunning with
       large dark eyes and a beautiful high-bosomed figure";
       "stunning photographs of Canada's wilderness areas"

The Jargon File (version 4.4.7, 29 Dec 2003):

stunning
 adj.

    Mind-bogglingly stupid. Usually used in sarcasm. “You want to code what in
    Ada? That's a ... stunning idea!”