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