The contemptuous nonchalance of her trailed hand irritated
him.
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The noun is nonchalance.
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In a sense, his nonchalance: is the epitomy of cool or; could be
unemotional or; simply misunderstands the gravity of the
problem.
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One might use nonchalance when dealing with someone in order to
give the impression that one doesn't mind either way in which a
situation is handled. Nonchalance means to be indifferent about
something and to be neutral.