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]WordNet (r) 3.0 (2006):
mettlesomeness n 1: courageous high-spiritedness