From Longman Dictionary of Contemporary Englishon your mettleon your mettleBritish EnglishPREPARE if you are on your mettle, you are ready to try as hard as possible because your abilities are being tested We’ll have to be on our mettle from the start.keep/put somebody on their mettle This was just his way of keeping me on my mettle. → mettle
Examples from the Corpus
on your mettle• He had been on his mettle throughout.• He thinks this keeps him on his mettle, but it often leaves him at a loss for the right word.• I see now that it was just his way of keeping me on my mettle and making sure I didn't get big-headed.• Variations in response put you on your mettle. 