Nonchalantly means in a calm unbothered way, as though nothing mattered. Often when a great deal does.
It attaches to actions rather than states, so somebody shrugs nonchalantly, strolls nonchalantly or nonchalantly mentions something enormous. That last construction is where the word earns its keep, since the calm is usually a performance and everybody can tell. It came from French with the spelling intact, which is why the ch is pronounced like sh. The noun is nonchalance.
1adverbnonchalantlyIn a casually unconcerned manner.
“He nonchalantly mentioned that he had sold the company.”
Old French
nonchaloir — to be unconcerned
French
nonchalant
English
nonchalant
English, today
nonchalantly
From nonchalant + -ly. nonchalant French nonchalant, from Old French nonchaloir (“to be unconcerned”), from non- + chaloir, from Latin - non + calēre.
Etymology from Wiktionary, CC BY-SA