Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

supererogatory
     adj : more than is needed, desired, or required; "trying to lose
           excess weight"; "found some extra change lying on the
           dresser"; "yet another book on heraldry might be
           thought redundant"; "skills made redundant by
           technological advance"; "sleeping in the spare room";
           "supernumerary ornamentation"; "it was supererogatory
           of her to gloat"; "delete superfluous (or unnecessary)
           words"; "extra ribs as well as other supernumerary
           internal parts"; "surplus cheese distributed to the
           needy" [syn: {excess}, {extra}, {redundant}, {spare}, {superfluous},
            {supernumerary}, {surplus}]