If you call someone pea-brained, you are saying they are stupid — as if their brain were as small as a pea. It is a rude, informal insult, not a careful or scientific word. People spit it out when they are angry, or when they make fun of a foolish plan: 'That was a pea-brained idea.' Though actually it is not only for people. You can call a decision pea-brained too, meaning the idea itself is silly. It paints a picture rather than giving a fair judgement, so it sounds like a slap, not like a teacher's note. You will hear it in casual speech and in stories. You should not use it about a classmate, a little brother, or anyone sitting next to you if you want to be kind.