DuckCorp Dico

Found one definition

1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   Supererogatory \Su`per*e*rog"a*to*ry\, a.
      Performed to an extent not enjoined, or not required, by duty
      or necessity; as, supererogatory services. --Howell.
      [1913 Webster]