can't punch one's way out of a paper bag
Be inept, as in Ask him to program the VCR? He can't punch his way out of a paper bag. This hyperbolic term for extreme ineptitude originally was an expression of contempt for a weak or cowardly boxer. [Slang; c. 1910]
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.