Is extremely stupid. This jocular phrase implies that the person so described couldn't perform the simplest of tasks. The full phrase is "couldn't pour water out of a boot if there was instructions on the heel." I'm not surprised that Zack took the train home and left his car in the parking garage at work—he couldn't pour water out of a boot! That fool couldn't pour water out of a boot if there was instructions on the heel! Dolt!