- couldn't pour water out of a boot (if there was instructions on the heel)
- couldn't pour water out of a boot (if there was instructions on the heel)Rur. stupid. •
I won't say Jim is dumb, but he couldn't pour water out of a boot.
•Jane couldn't pour water out of a boot if there was instructions on the heel—and she's the smartest one in her family!
Dictionary of American idioms. 2013.