Do their worst - Idioms by The Free Dictionary
do (one's) worst (redirected from do their worst)
do (one's) worst
To do something to the fullest extent that one can. This phrase usually has a negative connotation. Once I was all suited up and in goal, I told the offensive players to do their worst.
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.