be (as) thick as two short planks
To be remarkably stupid, dimwitted, or obtuse. I was as thick as two short planks back in high school. Good thing I straightened out in college. Jen's new girlfriend is very nice, but she's 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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