Do them a mischief - Idioms by The Free Dictionary
do (someone) a mischief (redirected from do them a mischief)
do (someone) a mischief
To do something that harms someone else. Primarily heard in UK, Australia. You sure did your brother a mischief by including him in that dangerous game.
do someone (or yourself) a mischief injure someone or yourself. informal
ˌdo somebody/yourself a ˈmischief (British English, informal or humorous) hurt somebody/yourself: You could do yourself a mischief wearing such tight trousers!