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.
• Life brings sorrows and joys alike. It is what a man does with them – not what they do to him – that is the true test of his mettle. — Theodore “Teddy” Roosevelt (1858–1919) 26th President of the United States
• We are not built for the mountains and the dawns and aesthetic affinities, those are for moments of inspiration, that is all. We are built for the valley, for the ordinary stuff we are in, and that is where we have to prove our mettle. Spiritual selfishness always wants repeated moments on the mount. We feel we could talk like angels and live like angels, if only we could stay on the mount. The times of exaltation are exceptional, they have their meaning in our life with God, but we must beware lest our spiritual selfishness wants to make them the only time. — Oswald Chambers (1874-1917) Scottish Baptist evangelist
Comment: Said to be an essential quality of a United States Marine.