A simple way of oen person expressing to another they are not interested in them - or their situation.
Who fucking cares if you got screwed by the car dealership - you're stupid for shopping there!
A more strong and vulgar term compared to "no one cares". If someone says this to you, you better shut the fuck up.
Person 1: Do you know how old is that waiter over there? Lemme tell you, its...
Person 2: Who fucking cares? I don't.