The Collaborative International Dictionary of English v.0.48:

Mettlesome \Met"tle*some\, a.
   Full of spirit; possessing constitutional ardor; fiery; as, a
   mettlesome horse.
   [1913 Webster] -- Met"tle*some*ly, adv. --
   Met"tle*some*ness, n.
   [1913 Webster]