be on your mettle
be on (one's) mettle
To be determined to succeed and thus prove one's worth, often in a difficult or unpleasant situation. I know my employees think I'm too young to be their supervisor, so I have to be on my mettle every day at the office.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
be on your mettlebe ready or forced to prove your ability to cope well with a demanding situation.
Farlex Partner Idioms Dictionary © Farlex 2017