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