Be (as) thick as two short planks British informal simile
Sometimes, The idiom can be shortened like 'Two short planks'.
An expression that describes how stupid someone is,
This machine is beautiful, but it's as thick as two short planks for one of us.
Sometimes you are thick as two short planks.
Used to describe someone who is naive, gullible, inexperienced, easily fooled, ignorant, unsophisticated, etc.
To say, or believe something or someone that seems completely crazy, delusional, or stupid
someone who is stupid or crazy
Thick 'is the UK secondary meaning of the word in this instance: to be stupid. Since the seventeenth century, this specific use has been popular. It appears that this particular connection to pieces of stubby wood began in the 1970's. As a 'plank' for a stupid person is also a UK term, there seems to be an additional level of stupidity inferred in the doubling of terms.
1. A pill or tablet that is hard for someone to swallow because of its large size
2. An fact, claim, proposal, etc. that is very difficult to believe or accept
1. The vet gave my adorable puppy a horse pill and he spat it out immediately.