(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.

(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.