
A rod/stick to beat somebody with
Negative Connotation
This phrase usually implies criticism and is often used to point out someone's faults or weaknesses. 
Metaphorical Use
The 'rod' or 'stick' is not a physical object but a metaphor for rebuke or reprimand. 
Not Literal
Don't confuse this with actual violence. It is a figurative phrase and does not involve physical beating. 