Source : Webster's Revised Unabridged Dictionary (1913)

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

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.

   2. Behind; in the rear. --Knolles.

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

   {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.

Source : WordNet®

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"