"Basement" is not usually bad in these cases. The image that comes to mind is a finished basement with a separate (or at least semi-private entrance).
I think "basement" has become synonymous with these cases because:
1. It reflects that the person is at least semi-independent (as opposed to children who live within the same space as the parents)
2. It is a bit insulting, reflecting society's view of the situation (it subtly implies antisocial behavior... The basement gamer, for example)
I think "basement" has become synonymous with these cases because: 1. It reflects that the person is at least semi-independent (as opposed to children who live within the same space as the parents) 2. It is a bit insulting, reflecting society's view of the situation (it subtly implies antisocial behavior... The basement gamer, for example)