Definitions: (1) full of spirit; courageous; brisk; fiery; (2) possessing constitutional ardor
Note: Not to be confused with meddlesome: to interfere in the affairs of another without their permission.

Derivation: a variant spelling of metal, used in a figurative sense. In the 17th-century the adjective mettlesome was popularly used as a reference for spirited horses.

Comment: Said to be an essential quality of a United States Marine.