Source : Webster's Revised Unabridged Dictionary (1913)

Mettle \Met"tle\, n. [E. metal, used in a tropical sense in
   allusion to the temper of the metal of a sword blade. See
   {Metal}.]
   Substance or quality of temperament; spirit, esp. as regards
   honor, courage, fortitude, ardor, etc.; disposition; --
   usually in a good sense.

         A certain critical hour which shall . . . try what
         mettle his heart is made of.             --South.

         Gentlemen of brave mettle.               --Shak.

         The winged courser, like a generous horse, Shows most
         true mettle when you check his course.   --Pope.

   {To put one one's mettle}, to cause or incite one to use
      one's best efforts.