METTLE

heart, mettle, nerve, spunk

(noun) the courage to carry on; “he kept fighting on pure spunk”; “you haven’t got the heart for baseball”

Source: WordNet® 3.1

Etymology

Noun

mettle (usually uncountable, plural mettles)

A quality of endurance and courage.

Good temperament and character.

(obsolete) Metal; a metallic substance.

Synonyms

• (quality of endurance and courage): courage, heart, spirit

Source: Wiktionary

Met"tle, n. Etym: [E. metal, used in a tropical sense in allusion to the temper of the metal of a sword blade. See Metal.]

Definition: Substance or quality of temperament; spirit, esp. as regards honor, courage, fortitude, ardor, etc.; disposition; -- usually in a good sense. A certain critical hour which shall... try what mettle his heart is made of. South. Gentlemen of brave mettle. Shak. The winged courser, like a generous horse, Shows most true mettle when you check his course. Pope. To put one one's mettle, to cause or incite one to use one's best efforts.

Source: Webster’s Unabridged Dictionary 1913 Edition