Stuff; spirit; constitutional ardour.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
Ardor or spirit, especially as regards honor, etc.; disposition; courage.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
Natural ardour; spirit; sprightliness; courage; warmth of temperament; substance.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
Ardor or keenness of temperament: spirit: sprightliness: courage.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Mettled, mettlesome.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)

Usage examples:

I am not so foolish as to threaten a man of your mettle
- The Early Short Fiction of Edith Wharton, Part 2 (of 10) by Edith Wharton
If the Rajah were on his mettle so undoubtedly was he.
- The Keeper of the Door by Ethel M. Dell
Hope was a great nut with the foils and she and the Frenchman had veritable battles, during which the little man, on his mettle and very excited, would squeal exactly like a rabbit.
- Fanny Goes to War by Pat Beauchamp