thick as a short plank

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

*thick as a short plank

 and *thick as two short planks
exceptionally 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!
See also: plank, short, thick
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • (as) thick as a short plank
  • plank
  • in the short haul
  • over the short haul
  • over the short term
  • short on (something) (and long on something else)
  • come short of
  • come short of (something)
  • be short of (something)
  • for short