Under somebody’s thumb
Definition 1 of 1
Pro Tip 1/3Power Imbalance
Indicates a significant power imbalance, where one person is much more powerful than the other. 
Ever since the merger, the staff has been under the new manager's thumb.
Pro Tip 2/3Negative Connotation
This phrase typically carries a negative connotation, highlighting a lack of freedom or autonomy. 
I don't like how she's under her friend's thumb—she can't make her own decisions.
Pro Tip 3/3Not Literal
It's a metaphorical phrase and doesn't involve physical control—it's about influence or psychological power. 
He's under his coach's thumb, following every bit of advice without question.