From figurative use of metal (since 16th century).

• IPA: /ˈmɛtəl/

mettle (uncountable)

1. A quality of courage and endurance.

   Synonyms: heart, pith, spirit

   • 1599 (date written), William Shakespeare, “The Life of Henry the Fift”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act IV, scene viii], page 90 ↗, column 2:

     By this Day and this Light, the fellow ha's mettell enough in his belly.

   • 2001, Harry J. Alexandrowicz, Testing your Mettle: Tough Problems and Real-world Solutions for Middle and High School Teachers, page xiii:

     Please read on and discover the issues in education that test the mettle of those who experience this world every day.

   • 1919, Henry B[lake] Fuller, “Cope Goes A-Sailing”, in Bertram Cope's Year: A Novel, Chicago, Ill.: Ralph Fletcher Seymour, The Alderbrink Press, →OCLC ↗, page 154 ↗:

     He was presently in as active circulation, on the campus and elsewhere, as ever. The few who looked after him at all came to the view that he possessed more mettle than stamina.

2. Good temperament and character.
   • c. 1601–1602 (date written), William Shakespeare, “Twelfe Night, or What You Will”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act V, scene i], page 274 ↗, column 2:

     Your Maſter quits you: and for your ſeruice done him, / So much againſt the mettle of your ſex

   • 1852 March – 1853 September, Charles Dickens, “Interlopers”, in Bleak House, London: Bradbury and Evans, […], published 1853, →OCLC ↗, page 328 ↗:

     The arrival of this unexpected heir soon taking wind in the court, still makes good for the Sol, and keeps the court upon its mettle.

3. Obsolete spelling of metal
   • 1624, Democritus Junior [pseudonym; Robert Burton], The Anatomy of Melancholy: […], 2nd edition, Oxford, Oxfordshire: […] John Lichfield and James Short, for Henry Cripps, →OCLC ↗:

     They have neither gold nor silver of their owne, wine nor oyle, or scarce any corne growing in those united Provinces, little or no Wood, Tinne, Lead, Iron, Silke, Wooll, any stuffe almost, or Mettle; and yet Hungary, Transilvania, that bragge of their mines, fertile England cannot compare with them.

• French: prouesse
• German: Mut, Eifer, Enthusiasmus, Stamina
• Portuguese: firmeza
• Russian: пыл

• German: Feurigkeit

mettle

1. (Scotland) Spirited, vigorous, stout-hearted.
   • 1902, John Buchan, The Outgoing of the Tide:

     And then one afternoon in the hinder end of April came young Heriotside riding to the Skerburnfoot. His arm was healed, he had got him a fine new suit of green, and his horse was a mettle beast that well set off his figure.