airhead

noun

Etymology: From air + head, in the sense of having one’s head filled with air instead of a brain; compare empty-headed.

1. A foolish, silly, or unintelligent person.

   “Sometimes people tell you directly that you aren't very smart, but most of the time they ignore your attempts to display your intelligence, or they kid you about being a "space case" or an "airhead." After a while you are bound to see yourself as not very intelligent, particularly if the people who have been sending you those messages about yourself are important to you.”

   “Yeah. Well, Tammi is a bit of an airhead, I guess, but I feel sorry for her.”