A neurological condition in which a woman's brain migrates to her breasts. This results in the patient having large boobs and empty space where her brain should be. This is the official medical term, but is more commonly referred to as air head or ditz when applied to a woman with big tits.
Friend: That hot girl at taco bell turned out to be fucking stupid.
Me: She suffers from brainboobitis.
Friend: Oh, I see.