This means that someone thinks the other person is "mean". You can look that up in a dictionary. It indicates someone is cruel, harsh, and not kind or nice.

They are asking you , how badly you can talk to / act towards another person. They are asking your moral limit- how mean CAN you be to someone else? They might be sarcastic. If they are being sarcastic, then they are saying you are acting very badly to another person. If you know this person well, they may be using this dramatic language to tell you they don't like how you acted.

It means what ever you do, it would be just for your own selfish motives. That also mean you are cruel and unkind. Cheers:)))

This is not something you would probably say to someone unless you knew them very well or you wanted to get into a fight. Perhaps it would be better to say, "Why are you being so mean?" rather than to flatly state it the way you did.