A person who's (mentally) fucked beyond repair.
A: How's your brother in law, I heard he got promoted.
B: Couldn't tell ya, havn't spoken to that defuct in years.
A high-severity defect that leaves the system in a very bad state. (noun)
Etymology: a combination of "defect" and the adjective form of a common 4-letter slang word derived from the latin fvccant which, in common parlance means to have been cheated.
Alternate definition: an unlikely condition of having regained ones virginity (but this would probably require a homophonic spelling).
I logged two defects on the layout and a defuct about the server crash.