Is stupidest a word?

Yes, "stupidest" is a real word and it is grammatically correct. I only found that out now as I researched this answer.

When I was in elementary school in the 1960's, I was also taught that one had to say:

1. stupid
2. more stupid (comparative)
3. most stupid (superlative)

In recent years, I hear people saying things like, "That's the stupidest thing I ever heard." Or, "Can anything be stupider than that?"

It seemed to me like slang, like people were just too lazy to speak correctly in the vernacular. However, when I looked it up on Google just now, I found the following in Grammarly:

Stupid is a two-syllable adjective, which means you can make its comparative and superlative forms in one of two ways: by adding the corresponding suffix or by adding more or most.

For some two-syllable words, says Grammarly, we put more and most in front of them but for stupid we can use either method.

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Comparative: er: tall, taller; stupid, stupider; more stupid

Superlative: est: tall, taller, tallest; stupid, stupider, stupidest; most stupid

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According to Grammarly:

Stupider is grammatically correct, it is a real word, and it’s been in use for at least the last two hundred years.

...

As with stupider, stupidest is perfectly correct and listed as standard in dictionaries.

I'm not going to argue with Grammarly and standard dictionaries. In fact, I like the words stupider and stupidest. There's just nothing that expresses disgust for lack of thought better than those two words, in my opinion.