English Idioms and Phrases: Over a barrel
To be over a barrel means to be out of control or under the control of someone else, for example in a bad situation and there is nothing you can do to get out of it, meaning you are helpless.
"I signed the contract and now I have to do the job, he has me over a barrel"
This phrase is another with a nautical origin, like 'get a word in edgeways'.
When a person had been close to drowning they were positioned over a barrel to help them get the water out of their lungs. Also when they were whipped they were often held in the same position, thus it gives the impression of being under someone else control and helpless.
Alternatives or Synonyms
Helpless, being under someones control