Nonchalance is the air of being calm, cool, and not bothered. The person may care inside, or may not; what you see is the surface: no fuss, no worry on the face, as if the thing in front of them is no big deal. We often talk of someone doing something with nonchalance — walking into a hard exam, shrugging at bad news, answering a sharp question as if it were small talk. It can be real calm, or it can be a show. In English it can sound a little stylish, and sometimes a little cold, depending on the moment. It is more at home in writing and in careful speech than in the playground. English borrowed the word whole from French; the feeling is “not heating up about it.”

漫不經心、若無其事