The Collaborative International Dictionary of English v.0.48:

Aback \A*back"\ ([.a]*b[a^]k"), adv. [Pref. a- + back; AS. on
   b[ae]c at, on, or toward the back. See Back.]
   1. Toward the back or rear; backward. "Therewith aback she
      started." --Chaucer.
      [1913 Webster]

   2. Behind; in the rear. --Knolles.
      [1913 Webster]

   3. (Naut.) Backward against the mast; -- said of the sails
      when pressed by the wind. --Totten.
      [1913 Webster]

   To be taken aback.
      (a) To be driven backward against the mast; -- said of the
          sails, also of the ship when the sails are thus
          driven.
      (b) To be suddenly checked, baffled, or discomfited.
          --Dickens.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Aback \Ab"ack\ ([a^]b"ak), n.
   An abacus. [Obs.] --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aback
    adv 1: having the wind against the forward side of the sails;
           "the ship came up into the wind with all yards aback"
    2: by surprise; "taken aback by the caustic remarks"