The word stupid is two syllables long. As an adjective, it can take both comparative and superlative forms. Stupid can take these forms either with a suffix (-er or -est) or with a preceding word (more or most). Both constructions are correct.
Stupider or More Stupid?
To determine how to create the comparative form of an adjective, you have to look at the number of syllables. For adjectives that are one syllable long, you can generally affix -er to the end to make them comparative. For example, because the word fat is only one syllable and its comparative form is fatter.
You cannot, however, add -er to an adjective that is three syllables or more. Instead, these adjectives become comparative with the addition of a preceding more. So, the word intelligent becomes more intelligent instead of intelligenter.
This brings us to two-syllable adjectives. They exist in between the hard-and-fast rules above. Some use -er, and some use more. For example:
- Happy becomes happier
- Lively becomes livelier
- Tangled becomes more tangled
- Bitter becomes more bitter
So which is stupid? Many writers feel uncomfortable using stupider, which can feel … well, not very smart. They opt for more stupid. However, it turns out that dictionaries and academics favor stupider. Indeed, this word has been in common usage for two centuries!
However, there is nothing grammatically incorrect about more stupid.
Stupidest or Most Stupid?
Every rule we outlined above for comparative adjectives remains in place for superlative forms.
With one-syllable adjectives like fat, you affix an -est suffix. With adjectives of three or more syllables, you add most before them.
Similarly, two-syllable adjectives can go either way. As with stupider, stupidest is the acknowledged standard superlative. Still, you can use most stupid without violating any grammar rule.
You should note, however, that the word stupid is an incredibly insulting pejorative. It would be best for you not to use it in your formal writing at all.
This polluted air also makes us stupider, slowing us down on cognitive tests and making it easier to get distracted. – Fast Company
Rich Lowry called the matter … perhaps the “stupidest and most unworthy controversy of the year.” –Slate