A phrase which once meant "anything excluding" or "not" but now which means absolutely nothing and is used by Yankees to indicate a lack of intelligence unbeknown to them.
Paul (From Tennessee): I got an 11 on my ACT...
Brutus (From New York): I got a 35, I am all but intelligent.
Paul: Are you sure they didn't mix up our tests?
Brutus: Oh wait, I didn't even take the ACT.
Two people that aren't married, but are "all but" married. They have children, love each other, and live the typical lifestyle of a typical married couple.
Commonly, they will have both been married-divorced before, and have subsequently decided that marriage is almost completely unnecessary, outside of possible tax and health-care benefits.
I'd like you to meet my 'all buts', Crystal. We have one child together, and two by our previous marriages. As a 33 year old man with children I find it pretty lame (and childish) to refer to her has my girlfriend.