(as) thick as a plank
Remarkably stupid, dimwitted, or obtuse. I smoked a lot of marijuana when I was in high school, so I turned out thick as a plank by the time I finally graduated. Jen's new girlfriend is very nice, but have you had a conversation with her? She's as thick as a plank. You took apart the entire washing machine just to unblock the pump? I swear, sometimes you can be as thick as a plank, Larry.
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