·noun Substance or quality of temperament; spirit, ·esp. as regards honor, courage, fortitude, ardor, ·etc.; disposition;
— usually in a good sense.
The semen. To fetch mettle; the act of self pollution. Mettle is also figuratively used for courage....
Dictionary of The Vulgar Tongue by Francis Grose
·noun Inborn mettle or courage; one's own temper. ...
Webster's Dictionary of the English Language