It's like saying 'fuck off' but normally used as an extremely serious manner. British term that although does not posses any swear words has a deep meaning.
Dave: Hey man sorry I broke your bike
James: I can't get it fixed, and that bike was a gift from my dad before he left
Dave: It's no big deal, calm down. You need to stop getting so stressed about everything.
James: You know what Dave, stuff off
