Boo boo swamp
The last bit of liquid at the bottom of a bottle or can, consisting mostly of backwash.
Dude: "Hey man, can I get a swig of that beer?"
Dude's friend: "It's just boo boo swamp."
Dude: "Is it cold?"
Dude's friend: "It's just boo boo swamp."
Dude: "Is it cold?"