chalant

adj. (slang, humorous) denoting a person or behaviour that is serious or formal; concerned with the matter at hand; not casual A fallaciously derivative antonym of the word nonchalant. "Nonchalant" is actually a standalone word borrowed from the French "nonchaloir" meaning "unconcerned". We never took the word "chaloir", however, and turned it into "chalant". We could have done, but we didn't. All that said, treating that first syllable as a prefix (which it was in the French) and extrapolating that chalant must be an antonym of nonchalant is actually reasonably logical and reflects the historically procedural manner in which the English language is developed. Nonetheless, at this point in time the word is only legitimate as a form of humorous slang.