1. Courage and fortitude.
2. Disposition or temperament.
Origin
Mettle was used interchangeably with the material metal until the early 1700s and it continued to be used in the figurative sense.
Quotes
Who is so ignorant as not to know that knights-errant are beyond all jurisdiction, their only law their swords, while their charter is their mettle and their will is their decrees?
— Miguel de Cervantes Saavedra, Don Quixote
