If someone were talking shit to you or to someone you know and it gets back to you. Acting like you don't care or not worrying about it is having your "shit shield" up.
Nick: "Hey Bruce, James has been talking shit about you all day. He just showed up for the party man. You'd better have your shit shield up."
Bruce: "I know. But I don't care. It does not bother me unless he gets into my face about it."