[1] : The Collaborative International Dictionary of English v.0.48

aback \aback"\ .aba^k", adv. pref. a- + back; as. on
   baec 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
see also:
back to be taken aback 

[2] : The Collaborative International Dictionary of English v.0.48

aback \ab"ack\ a^b"ak, n.
   an abacus. obs. --b. jonson.
   1913 webster

[3] : WordNet (r) 2.0

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"

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