nonchalant
Concise Oxford English Dictionary © 2008 Oxford University Press:
nonchalant /ˈnɒnʃ(ə)l(ə)nt/
▶adjective casually calm and relaxed.
– derivatives
nonchalance noun,
nonchalantly adverb.
nonchalance noun,
nonchalantly adverb.
– origin C18: from Fr., lit. ‘not being concerned’, from the verb nonchaloir.
'nonchalant' also found in these Oxford entries:

