His train of thought is still boarding

If you were half as smart as you think you are, you’d be twice as smart as you really are.

It's impossible to underestimate you.

One of my personal favorites:"They only got two brain cells and both of them are fighting for third place."

Ahh yes, “it’s better to be silent and thought a fool than to speak and remove all doubt”. A classic.

In Greece we say "when it was raining brains, you had an umbrella."

You're the reason we have warning labels.

He needs to carry a plant to make up for the oxygen he's wasting.

I can explain it again, if you'd like, but I can't understand for you.

If your brains were dynamite there wouldn't be enough to blow your hat off.

Once heard someone say "Well he's about as sharp as a sock full of soup".

You could blow in their ear and they would say thanks for the refill.

"If you ever had a clever thought, it died alone and afraid."

"Well pardon me, ma'am but what you don't know could fill a warehouse."

Unencumbered by the thought process.

I would agree with you but then we would both be wrong.

Saw two coworkers going about a task in the worst way I could imagine. I said "I believe if there was a harder way to do that you two would find it."They didn't get it.

"At this point, you can only impress me."

One of my all time favorites is "More foam than beer."

In russian we have "intelligent thoughts have always followed him, but he was faster."

Couldn't pour water out of a boot with instructions on the heel.

The wheels spinning, but the hamsters dead.

I think I saw this one here previously “You aren’t the biggest idiot in thel world but you better hope they don’t die”.

A village somewhere is missing their idiot.

My teacher said I was as "quick as a glacier"...

He's so far behind he thinks he's first.

"I'm guessing you weren't burdened with an overabundance of schooling."

Now I know what Douglas Adams was talking about.“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”

These euphemisms telling people they are stupid, will cut straight to the chase.People from all around the world shared various ways to insult stupid people politely by using both modern ingenuity and old folk proverbs.

30 Polite Ways To Describe A Person’s Lack Of Intelligence 

You could hide your own Easter eggs.

30 Polite Ways To Describe A Person’s Lack Of Intelligence

Understand yourself and those around you.

#30

Similar ideas to #30

Optimal choices

3. Adjust When It’s Necessary

Ignorance is Bliss