nonchalance       
noun [ U ] uk/ˈnɒn.ʃəl.əns/ us/ˌnɑːn.ʃəˈlɑːns/
 若无其事;漠不关心;毫不在乎
calm behaviour that suggests you are not interested or do not care
  He leaned back in his chair with apparent nonchalance. 他往椅背上靠了靠,一副满不在乎的样子。 
 I may feign nonchalance on the phone to my mum, but actually I am terrified. 我可能在电话里假装对我妈妈漠不关心,但实际上我很害怕。 
 See: nonchalant
 - More examples
- She expressed nonchalance about the 40 percent chance she will need another operation.
- I affected a nonchalance wholly at variance with my real feelings.
- Jason's studied nonchalance disappears when he discusses the pride he takes in his work.