Etymology From figurative use of metal (since 16th century). The two spellings were originally interchangeable variants, but came to be distinguished by sense in the 18th century.
1
Usually A quality of courage and endurance.
By this Day and this Light, the fellow ha's mettell enough in his belly.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:
Please read on and discover the issues in education that test the mettle of those who experience this world every day.2001, Harry J. Alexandrowicz, Testing your Mettle: Tough Problems and Real-world Solutions for Middle and High School Teachers, page xiii:
2 more examples
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.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:
Its possible that some traders will become less interested as their offline lives resume more fully, but their mettle may really be tested when the market runs into its next downturn.2021 April 28, Tara Siegel Bernard, “Trading Stock Tips on TikTok, Newbies Are Deeply Invested in Learning”, in The New York Times, →ISSN:
2
Usually Good temperament and character.
Your Maſter quits you: and for your ſeruice done him, / So much againſt the mettle of your ſexc. 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:
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.1852 March – 1853 September, Charles Dickens, “Interlopers”, in Bleak House, London: Bradbury and Evans, […], published 1853, →OCLC, page 328:
3
Obsolete, Usually Obsolete spelling of metal (“metallic substance”).
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.1624, Democritus Junior [pseudonym; Robert Burton], The Anatomy of Melancholy: […], 2nd edition, Oxford, Oxfordshire: […] John Lichfield and James Short, for Henry Cripps, →OCLC:
Etymology From figurative use of metal (since 16th century). The two spellings were originally interchangeable variants, but came to be distinguished by sense in the 18th century.
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.1902, John Buchan, The Outgoing of the Tide: