a stick to beat someone with
US /ə stɪk tə biːt ˈsʌm.wʌn wɪθ/
UK /ə stɪk tə biːt ˈsʌm.wʌn wɪθ/

1.
a piece of information or an argument that can be used to criticize or harm someone
:
•
His past mistakes became a stick to beat him with during the election campaign.
•
Don't give them a stick to beat you with by admitting your weaknesses.