airbrain

airhead

A dim-witted, useless, or unintelligent person. I can't believe I got paired with such an airhead for this assignment. He's not going to get any work done at all!
Farlex Dictionary of Idioms.

airhead

and airbrain
n. a stupid person. (Someone with air where there should be brains.) Some airbrain put mustard in the ketchup dispenser. Bart seems like such an airhead, but he’s all heart.

airbrain

verb
See airhead
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • airhead
  • airheaded
  • pumpkin head
  • (one's) way out of a paper bag
  • out of a paper bag
  • (one's) way out of a wet paper bag
  • couldn't find (one's) way out of a paper bag
  • smurfbrain
  • dim bulb
  • dim