The quality or state of being cavalier; an attitude of dismissive arrogance, recklessness, or disregard for serious matters.
From 'cavalier' (from Italian 'cavaliere,' meaning horseman or knight) plus the suffix '-ish' (meaning resembling or somewhat like) plus '-ness' (forming abstract nouns). The term evolved from noble riders to describe aristocratic indifference.
The British Civil War saw 'Cavaliers' (royalists) versus 'Roundheads,' and 'cavalierishness' became shorthand for the aristocratic arrogance that helped topple a king—it's a word carrying the weight of a revolution.
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