get (one) over a barrel
To cause one to be in a position of vulnerability, weakness, or powerlessness. Now that we know about his involvement, we've got him over a barrel—he'll give us whatever we want to buy our silence.
get someone over a barrel
get someone in a helpless position; have someone at your mercy. informal This phrase perhaps refers to the condition of a person who has been rescued from drowning and is placed over a barrel to clear their lungs of water.