"Out of a paper bag" or "out of a wet paper bag" is a common expression to indicate someone's extreme inability to perform a particular task.
The phrase originated in the world of boxing. "He couldn't punch his way out of a paper bag" was a derisive way to indicate the weakness of an opponent. A paper bag is very flimsy, and if a person happened to be trapped inside a huge, life-sized paper bag, he should be able to easily tear himself free. Anyway, over time, the "paper bag" analogy spread and was applied to activities outside the boxing ring.
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