The superlative form of stupid is most stupid, as in I made the most stupid mistake, not stupidest. Stupidest can be heard and seen everywhere, including many modern dictionaries, but it’s wrong. (Please don’t ask me to roll out my entire exposition on why finding a word in a modern dictionary doesn’t make it legitimate, standard English. Paul and I have already paddled across that ocean several times.)
Stupid is just like lucid (same –id ending). The comparative form is more lucid, and the superlative form is most lucid. Likewise, the comparative form of stupid is more stupid, and the superlative form is most stupid. In fact, in general, –id words use more and most instead of –er and –est. The water was more tepid, he was the most lucid, they could have been more candid, his reflexes have grown more torpid, the milk was the most rancid, his tongue had grown more acrid. No one would even think to say tepidest, lucidest, candidest, torpidest, rancidest, or acridest–I hope.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 
Now this is new to me! And I’m no dummy when it comes to correct grammar. I frequently pick out errors in newspapers, advertisements, & even in scientific journals so to read your comments about stupidest is very interesting – thank you! I like to pride myself on using the correct grammar. I found it amusing that when I typed ‘stupidest’, it didn’t show a red colour as spelling errors normally do. Shows how ubiquitous the word has become in our language. Oh, if you find spelling errors in my comments (like colour), it’s because I’m Canadian. And no, I never say “eh”. 🙂
Reply: Thanks, Judy. I’m not surprised at your spell checker. Mine likes the substandard words ANYWAYS and IRREGARDLESS, too. 🙂
Have you explained “why finding a word in a modern dictionary doesn’t make it legitimate, standard English”? What does make something standard English?
Reply: Yes, we have talked about it on a couple of different occasions.
Please don’t ask me to roll out my entire exposition on why finding a word in a modern dictionary doesn’t make it legitimate, standard English.
And the corollary to that: Just because you Google a word and turn up 2,000 instances of it doesn’t make it right!
Reply: Amen! 🙂 Common usage—even 2000 of them—isn’t necessarily correct usage.
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