Well, this word is tricky.  Better to begin with the etymology. 

Cavalier comes from the Latin word caballarius, meaning, a horseman.  Caballarius derives from the Latin word for horse, caballus.  Caballus is the common, Vulgar Latin word for horse.  It replaced the Classical Latin word, equus.

That is the easy part.  Now, things get weird. 

A cavalier, obviously, was once an armed horseman, a knight.  Later, a caviler was a gentleman of the court.  This makes sense.  Knights ended up at the court of the king more often than commoners.  In the mid 1600’s a Cavalier was a supporter of Charles the First. 

Cavalier, at some point, took a new job as an adjective.  Someone cavalier does not really care about you, or anything important.  They are haughty, perhaps even disdainful.  Someone cavalier is probably a snob, looking down on others.  They are on their “high horse.” 

These days, the most common cavalier you will see is the Chevy Cavalier.