What does it mean when somebody tells you to shut up?

“Shut up” is a direct command with a meaning very similar to “be quiet”, but which is commonly perceived as a more forceful command to stop making noise or otherwise communicating, such as talking. The phrase is probably a shortened form of “shut up your mouth” or “shut your mouth up”.

How do you respond to IDC?

”I don’t care!” ”I think you care.” If someone says they “don’t care,” they likely DO care. Not witty, but in practice it’s effective to just say “Fine.” And walk away.