put someone on their mettle (of a demanding situation) test someone's ability to face difficulties in a spirited and resilient way.

☞ Originally the same word as metal, mettle was no more than a variant spelling that gradually became particularly associated with figurative uses of the word, meaning 'quality of temperament', and from that 'natural spirit' or 'courage'. These senses eventually developed so far from the literal senses that it was no longer apparent that they were originally the same word. The distinctive spellings metal and mettle to distinguish the two were in use by the early 18th century, though not necessarily universally applied until the following century.