(as) thick as a short plank
Remarkably stupid, dimwitted, or obtuse. You're thick as a short plank if you think you can swim across that river. I must have been as thick as a short plank when I was younger, because I sure did some stupid things.
*thick as a short plank
and *thick as two short planksexceptionally dim-witted. (*Also: as ~.) Dumb? He's as thick as a short plank, more like. Oh, I'd not say she was stupid. As thick as two short planks, yes, but stupid? Never!