(as) thick as two 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 planks by the time I finally graduated. Jen's new girlfriend is very nice, but she's as thick as two planks.

thick as two planks

thick as two short planks

BRITISH, INFORMAL

If someone is as thick as two planks or as thick as two short planks, they are very stupid. His people regarded him as a great and wise king. In fact he was as thick as two planks.

thick as two (short) planks

very stupid. informal

Variants of this expression include thick as a plank and thick as a brick . There is a play on thick in its basic sense ‘of relatively great depth from side to side’ and its colloquial sense ‘stupid’.