She was very tense, but she preserved an appearance of nonchalance.

她内心很紧张，可外表却装着满不在乎的样子。

《新英汉大辞典》

But still with some resemblance of nonchalance I asked, ?" "how different?"

不过我还是漫不经心地问，“有何不同“

哈佛公开课 - 幸福课课程节选

Investors are more relaxed, but their nonchalance may prove misplaced.

ECONOMIST: The world economy

But if ordinary citizens begin to complain, as many may with the health-service cuts, nonchalance will become less easy.

ECONOMIST: A wild gallop to the right causes consternation