met·tle 
/ˈmɛtl̟/ noun
/ˈmɛtl̟/ noun [noncount] : strength of spirit : ability to continue despite difficulties
▪ He proved/showed his mettle [=he proved/showed how tough he is] as a fighter tonight. ▪ The competition will test her mettle.
▪ He proved/showed his mettle [=he proved/showed how tough he is] as a fighter tonight. ▪ The competition will test her mettle.
on your mettle
chiefly Brit : making an effort to do as well as possible
▪ She'll have to be on her mettle [=at her best] to win this competition.
▪ She'll have to be on her mettle [=at her best] to win this competition.
