Definition of do someone (or oneself) a mischief in English:

do someone (or oneself) a mischief

phrase

British
informal
  • Injure someone or oneself.

    ‘I would have done myself a mischief if I'd carried on’
    • ‘For God's sake, calm down before you do yourself a mischief!’
    • ‘But is there a chance he'll do himself a mischief?’
    • ‘She leapt out of her stretch position without doing herself a mischief that would be regretted later, and called the story in to the news desk.’
    • ‘‘Steady on,’ said a male voice from within, ‘you'll do yourself a mischief.’’