Over a barrel
Helpless, in someone's power.
This is an American phrase and first appeared in the mid-20th century. It is supposed that it alludes to the actual situation of being draped over a barrel, either to empty the lungs of someone who has been close to drowning, or to give a flogging. Either way, the position of helplessness and in being under someone else's control is what is being referred to.
The first reference to the phrase used with that meaning that I can find is rather later than might be expected - 1938. That's in a cartoon from the Pennsylvania newspaper The Clearfield Progress.
The cartoon writer's colloquial use of the phrase and the lack of any explanation of it implies that that the audience was expected to be familiar with it. Given that, we may yet find an earlier citation.
In the following year Raymond Chandler made what appears to be an punning reference to the phrase, in The Big Sleep - referring to a gun barrel:
"We keep a file on unidentified bullets nowadays. Some day you might use that gun again. Then you'd be over a barrel."