A friend that over the years you have become attached to for unknown reasons. It is usually someone you have invested a lot of time and money in, all in good fun, for a while. They always let you down but you can't seem to stop hanging with them or helping them. They are typically low lifers who take what they can get from anyone and leave everyone thinking they're a good friend, until they're gone.
"i thought debbie was gonna be there for me, turns out she's just my deadend friend."
"why bother go pick debbie up for the party? she's just a deadend friend."