DuckCorp Dico

Found one definition

1.                 From 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"