mettlesome (adj.)

1580s, a variant spelling of metal. Both forms of the word were used interchangeably (by Shakespeare and others) in the literal sense and in the figurative one of "stuff of which a person is made, (a person's) physical or moral constitution" (1550s), hence "natural temperament," specifically "ardent masculine temperament, spirit, courage" (1590s). The spellings diverged early 18c. and this form took the figurative sense. Related: Mettled.

word-forming element used in making adjectives from nouns or adjectives (and sometimes verbs) and meaning "tending to; causing; to a considerable degree," from Old English -sum, identical with some, from PIE root *sem- (1) "one; as one, together with." Cognate with Old Frisian -sum, German -sam, Old Norse -samr; also related to same.

"It usually indicates the possession of a considerable degree of the quality named: as mettlesome, full of mettle or spirit; gladsome, very glad or joyous" [Century Dictionary]. It is also, disguised, the ending in buxom. For the -some used with numbers (twosome, foursome, etc.), see -some (2).