IPA: /nɒnˈʃæl ənt li/
KK: /nɑnˈʃæl ənt li/
In a way that shows you are calm and relaxed, without worrying or being excited about something.
She nonchalantly walked into the room as if nothing was wrong.
Nonchalantly → It is formed from "non-" (meaning not) and "chalant" (from French *chaloir*, meaning to be concerned or to care). The word describes a manner of acting that is not concerned or not caring, hence showing a relaxed or indifferent attitude.
Think of someone who is 'not' ('non-') 'caring' ('chalant') about a situation — that's why they act in a nonchalant way.
No commonly confused words.