behaviour or play, especially of children, that causes trouble but not serious harm to other peoplebe up to/get up to mischief (=do something bad): The boys are always up to some mischief or other.
get into mischief (=start behaving badly): He got into mischief the moment he left school.
keep (someone) out of mischief (=stop (someone) behaving badly): The work will keep you out of mischief.