1. Someone who's head is devoid of nominative brain tissue. Similar to the more common air head the cranium of the subject contains nothing but gas. However in this instance, the gas inside the cranium consists mainly of stinky gut methane.
2. Someone who consistently brain farts.
"I admit that I slept with your best friend, honey but honestly, it didn't mean anything."
"Why don't you just fart off, fart brain?!"