Literally, when somebody's brain has stopped functioning such that they will never regain conciousness, hence coloquially somebody who is idiotic, dumb or clueless beyond belief. Similar to "too stupid to live".
Joe:Poor guy, he smoked way to much pot.
Bill:He's quite the vegetable.
Joe:Maybe we should wipe up the drool from his chin?
Bill:Naw,he is braindead,he will just keep drooling.
Shit got craxy at the party mansion. Next day Jono was wondering why he could not function when it occurded to him that he had braindeaded himself. Things are simpler for him now.
A increadibly idiotic individual who hasn't a sustance for anything that is going on. He or she is stupid, retarded, mental, ignorant, annoying, funny to make fun of, or any combination of the previous terms.
