On one’s mettle

To be on one’s mettle means to be ready and primed to perform at one’s best and the expression dates in this sense from the late 16th century. Mettle is in fact an alternative spelling of ‘metal’, which was used figuratively from the mid-1500s onwards to describe a man’s ‘courage, strength or spirit’. From the late 1500s, the alternative spelling of ‘mettle’ was adopted to differentiate this nobler concept from the strength of ordinary ‘metal’.