Over a barrel

What is the origin of phrase "to be over a barrel?"
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Apparently it is of American origin, and the most likely derivation comes from a form of punishment, or humiliation, involving binding someone's hands and feet and rolling them over a barrel.

A person in that situation is completely at the mercy of his captors/tormentors and thus is had 'over a barrel'.

DFG

According to my references (and www.phrases.org.uk meanings over-a-barrel.html on this site) it refers to a method of forcing water out of a near drowning victim's lungs.