nonchalant

adjective
  1. 1

    Casually calm and relaxed.

    โ€œWe handled the whole frenetic situation with a nonchalant attitude.โ€

  2. 2

    Indifferent; unconcerned; behaving as if detached.

    โ€œHe is far too nonchalant about such a serious matter.โ€

Source: https://en.wiktionary.org/wiki/nonchalant