1
general::
In a weak or difficult position, as in Once the competitors found a flaw in our product, they had us over a barrel. This slangy expression, first recorded in 1938, supposedly alludes to reviving a drowning victim by placing the body head down over a barrel and rolling it back and forth, so as to empty the lungs of water. The expression survives, although happily the practice does not.
American Heritage Idioms
2
general::
Phrase(s): *over a barrel
Fig. out of one’s control; in a dilemma. (*Typically: get someone ~; have someone ~; put someone ~.) • He got me over a barrel, and I had to do what he said. • Ann will do exactly what I say. I’ve got her over a barrel.
McGrawhill's American Idioms And Phrasal Verbs