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general::
Phrase(s): 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!
McGrawhill's American Idioms And Phrasal Verbs