A person's ability to cope well with difficulties or face challenges with courage; strength of character or spirit.
Variant spelling of 'metal' from Old French metal, from Latin metallum. The figurative sense developed from the idea of testing metal's quality through heating and hammering, applied metaphorically to human character.
The phrase 'test your mettle' perfectly captures the word's metallurgical origins - just as blacksmiths test metal's strength through fire and force, life tests human character through challenges. The spelling distinction from 'metal' helps preserve this beautiful metaphor.
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