Do their worst - Idioms by The Free Dictionary
do (one's) worst (redirected from do their worst)
do (one's) worst
To do something (typically something negative) to the fullest extent that one can. Often used as a challenging imperative. Do your worst, winter! My new coat can withstand your blizzards! They can do their worst, but I refuse to be intimidated.
do your worst do as much damage as you can (often used to express defiance in the face of threats).
do your ˈworst be as harmful, unpleasant, violent, etc. as you can: I refuse to pay this bill. Let them do their worst. ♢ The electricity company did their worst and cut off the supply.