thick as two short planks

(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.
See also: plank, short, thick, two
Farlex Dictionary of Idioms.

(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.
See also: plank, short, thick, two
Farlex Partner Idioms Dictionary
See also:
  • (as) thick as two short planks
  • (as) thick as two planks
  • thick as two planks
  • be (as) thick as a short plank
  • (as) thick as a brick
  • be (as) thick as a brick
References in periodicals archive
The Tamayuz colt was as thick as two short planks when making his debut over this course and distance at the beginning of last month but, having raced at the back of the pack early on, flew at the finish to take second to Jallota.
I'M still trying to work out whether I should be Ooohing and Aaahing over the reprieve of a calf that has become the Nation's Sacred Cow, or bloody insulted that this Government is treating the electorate like we're all as thick as two short planks.
IF ever we needed proof that - with a few notable exceptions - boxers are as thick as two short planks, here it is.
Is it to remind us her kids are as thick as two short planks? 5 Bernard Hogan-Howe's in London after learning in Liverpool how to defeat the enemy.
He looked as thick as two short planks when asked to go forward but, when the penny finally did drop, he shot to the front inside the last to score by three and a quarter lengths from Lord Ofthe Shadows.
Despite looking as thick as two short planks, Pastoral Player got the hang of what was needed entering the final furlong and wasn't seriously extended to beat Locksley Hall by a neck and earn a step up in class.