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

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”.

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

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.

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

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.

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.”

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

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

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

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

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

Unencumbered by the thought process.

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

You could hide your own Easter eggs.

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."

It's impossible to underestimate you.

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

You're the reason we have warning labels.

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

His train of thought is still boarding

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

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

The wheels spinning, but the hamsters dead.

A village somewhere is missing their idiot.

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 

Understand yourself and those around you.

#21

Similar ideas to #21

ALBERT EINSTEIN

The SEE-I method for explaining ideas

Ruthless prioritization