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"Cavalier" was a derogatory term for a royalist, characterized as a floofy dandy. I just learned this from the Revolutions podcast. (I'm now listening to the first season, about the English Revolution.)
I first heard of cavaliers from 1E AD&D, where they all have a minimum of 15 in STR, DEX, and CON, and have all of these defense and attack bonuses, immunity to a whole bunch of spells, and nice armor.
I have not revised my definition of them until just now.