It turns out stupider is an actual word. Stupider and more stupid are both comparative forms of the adjective stupid. They can be used interchangeably. In the English language, stupid is one of just a few adjectives that have two grammatically correct options for their comparative form. The superlative forms of stupid are stupidest and most stupid.
The word stupid is an adjective (a word that describes a noun). It refers to someone or something that’s foolish or senseless. If you want to compare two things that are stupid, you use the comparative form of stupid to describe which has a greater level of stupidity.
For example, you could say “Thing A is stupid, but thing B is stupider.” This would express that you find thing B to have a higher degree of stupidness than thing A. If you said “Thing B is more stupid than thing A,” it would mean the same thing as the first sentence. Both sentences show the comparative relationship between things A and B.
Comparative and Superlative Adjectives
When you want to compare two nouns, you can use a comparative adjective. For example, in the sentence “Sam is bigger than Sally,” the adjective bigger is comparative. It tells you that Sam is larger in size than Sally.
If you’re comparing more than two things, you can use a superlative adjective. Consider this sentence: “Sam is bigger than Sally, but John is the biggest.” In this case, John is larger in size than both Sally and Sam. This shows a progression in size among the three people, with Sally as the smallest, Sam in the middle, and John as the largest.
In most cases, comparative and superlative adjectives follow a simple pattern. You add the suffix -er to the original adjective to form the comparative. Add the suffix -est to form the superlative. This rule is true of almost all one-syllable adjectives. For example, cleanwould become cleaner and cleanest.
Some two-syllable adjectives (in particular, adjectives ending with -y , -er, -le, and -ow) also follow this rule. This includes dirty (which becomes dirtier and dirtiest), clever (cleverer and cleverest), little (littler and littlest), and narrow (narrower and narrowest).
In most other cases, adjectives with two syllables use the words more and most to create their comparative and superlative forms. For example, important and public are both adjectives with multiple syllables. Their comparative forms are more important and more public. Their superlative forms are most important and most public.
Stupider, More Stupid, and Other Exceptions
Stupid is a two-syllable adjective. Based on the rule above, it should use the words more and most to form its comparative and superlative forms, and it does. However, it also breaks this rule by using -er and -est as alternative forms.
A few other adjectives use -er and -est as well as more and most, such as clever, likely, and pleasant. This means both of these sentences are grammatically correct: “Betty is more likely to win the election than Bobby” and “Betty is likelier to win than Bobby.”