The Collaborative International Dictionary of English v.0.48:

Outmantle \Out*man"tle\, v. t.
   To excel in mantling; hence, to excel in splendor, as of
   dress. [R.]
   [1913 Webster]

         And with poetic trappings grace thy prose,
         Till it outmantle all the pride of verse. --Cowper.
   [1913 Webster]