Mettle is strength of character shown under test. Prove his mettle, test their mettle, show some mettle.
It appears almost only in a handful of phrases about being tested, and on your mettle means roused to do your best. It is the same word as metal, split off in spelling in the eighteenth century to keep the senses apart.
1nounmettleStrength of character shown under test.
“The final match was a real test of the young team's mettle.”
mettle is a strength.
METTLE → the METAL you turn out to be made of
What you are made of, and it only shows once the fire is under it, the way metal does.
hooks ontometal
A figurative use of metal, from the temper of a blade; the two spellings separated in the eighteenth century.
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.
Etymology from Wiktionary, CC BY-SA