a stick to beat someone with

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