thick as two short planks
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(as) thick as two short planks
Remarkably stupid, dimwitted, or obtuse. I smoked a lot of marijuana when I was in high school, so I turned out thick as two short planks by the time I finally graduated. Jen's new girlfriend is very nice, but she's as thick as two short planks.
(as) thick as two short ˈplanks(informal) (also (as) thick as ˈshit taboo, slang) (British English) (of a person) very stupid: Because she’s a model, people assume she’s as thick as two short planks, but she isn’t. OPPOSITE: (as) bright as a button
Thick is the opposite of thin and can also mean ‘stupid’ in informal language.