Stupid vs Shitbrain - What's the difference?

stupid | shitbrain |

As nouns the difference between stupid and shitbrain

is that stupid is a stupid person; a fool while shitbrain is (vulgar) a stupid person.

As an adjective stupid

is lacking in intelligence or exhibiting the quality of having been done by someone lacking in intelligence.

As an adverb stupid

is (slang|dated) extremely.