be on (one's) mettle(redirected from be on their 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.
be on your mettlebe ready or forced to prove your ability to cope well with a demanding situation.
be on, show, prove, etc. your ˈmettlebe prepared to do the best work you can or perform as well as you can in a particular situation: When the boss comes round, I want you all to show your mettle. ♢ He’ll have to be on his mettle if he wants to win the next race.
Mettle is the ability and determination to do something successfully in spite of difficult conditions.
See also: mettle