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general::
Phrase(s): *thick as a short plank [and] *thick as two short planks
exceptionally dim-witted. (*Also: as ~.) • Dumb? He’s as thick as a short plank, more like. • Oh, I’d not say she was stupid. As thick as two short planks, yes, but stupid? Never!
McGrawhill's American Idioms And Phrasal Verbs