Unconcerned means not worried by something that might worry other people. Calm, and possibly too calm.
It takes about or by. The word can be a compliment about steadiness or a criticism about indifference, and the same behaviour reads either way depending on how serious the situation is, which is why appearing unconcerned about a real danger is not admired. It also means uninvolved, so somebody unconcerned in an affair has no part in it, and that use is formal and mostly legal.
1adjectiveunconcernedNot worried or anxious about something.
“He seemed entirely unconcerned about the deadline.”
From un + concerned or unconcern + ed.
Etymology from Wiktionary, CC BY-SA