Mettle

... referring to a person's ability to cope well with difficulties; resilience, spirit, fortitude or strength of character under pressure. Key synonyms: Grit, fortitude, resolve, tenacity, pluck, backbone.

Common phrase: "To test one's mettle", to put someone's strength, courage or endurance to the test in a challenging situation.

"Mettle" originated as an alternate spelling of metal, 16/17th centuries, the two spellings were used interchangeably. Over time, figurative usage regarding the "temper" or "substance' of a person's character split into mettle, while the physical chemical elements retained the spelling 'metal'.

TRECHO

"You must be upon your mettle, for others are engaged in the same service and will exert themselves to the utmost to outdo you." -George Washington

"Life brings sorrows and joys alike. It is what a person does with them, not what they do unto the person, that is the true test of their mettle." -Theo Roosevelt

"You who live your lives in cities or among peaceful ways cannot always tell whether your friends are the kind who would go through fire for you but on the Plains one's friends have an opportunity to prove their mettle." William F. "Buffalo Bill" Cody