Someone described as nonchalant is relaxed and calm, either because they do not care about something or because they are not worried about something. Nonchalant can also be used to describe something, such as demeanor or behavior, that expresses such relaxed, calm unconcern.

// The team showed a somewhat nonchalant attitude at the beginning of the season, but they became more serious once the championship was within reach.

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"He is largely unaffected by the fame and fortune and all the talk of greatness tends to be greeted with a nonchalant shrug." — The Evening Times (Glasgow, Scotland), 19 Mar. 2024

Since nonchalant ultimately comes from words meaning "not" and "be warm," it's no surprise that the word is all about keeping one's cool. Nonchalant’s Old French ancestor is the verb nonchaloir, meaning "to disregard," which combines non-, meaning "not," with chaloir, meaning "to concern." Chaloir in turn traces back to the Latin calēre, meaning "to be warm" (calēre is also the forerunner of the heat-related English word calorie). You might assume that the prefix non- implies the existence of an antonymous chalant, but no such word has developed in English. It’s no big deal though—if you want a word that means the opposite of nonchalant, both concerned and interested can do the job.