(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. You took apart the entire washing machine just to unblock the pump? I swear, sometimes you can be as thick as two short planks, Larry.
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