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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.