I ask this question because Webster runs a lot of top 10 lists that interest me from time to time. The current list I was browsing is called "Ten Painless Ways to Improve a Conversation". The second word of the list is the adjective "cavalier", meaning "disregard" or "showing no concern".
This puzzled me since the noun means somebody who is a gentleman or a type of cavalryman. Why would the two words have such differing meanings? This led me to search for the etymology of cavalier, the noun having come into use in the 1600-1700s by the French and Englishmen.
The etymology for the adjective is confusing at best; here a quote from EtymologyOnline:
Sense advanced in 17c. to "knight," then "courtly gentleman," which led to adj. "disdainful" (1650s); earlier "gallant" (1640s).(snip)
How on earth could "courtly gentleman" lead to the adjective "disdainful"?
Best Answer
In the English Civil War (1642-1651), the Cavaliers were the supporters of the King, opposed to the Roundheads, who supported Parliament. Cavaliers wore their hair long and dressed in conspicuous finery; Roundheads cut their hair short (hence the name) and dressed very plainly.
The Roundheads won that war, and I suspect that the pejorative sense of "cavalier" arose as a result, especially since your first citation seems to have appeared around the end of the war.