From Longman Dictionary of Contemporary Englishhit somebody where it hurtshit somebody where it hurtsinformal to do something that you know will upset someone in the most damaging way Hit your husband where it hurts – in his wallet! → hit
Examples from the Corpus
hit somebody where it hurts• Instead of locking up drug offenders, hit them where it really hurts - in the wallet.• Tax day hits him hard, hits him where it hurts the most. 