1580s, from Italian cavalliere "mounted soldier, knight; gentleman serving as a lady's escort," from Late Latin caballarius "horseman," from Vulgar Latin caballus, the common Vulgar Latin word for "horse" (and source of Italian cavallo, French cheval, Spanish caballo, Irish capall, Welsh ceffyl), displacing Latin equus (see equine).
Sense advanced in 17c. to "knight," then "courtly gentleman" (but also, pejoratively, "swaggerer"), which led to the adjectival senses, especially "disdainful" (1650s). Meaning "Royalist adherent of Charles I" is from 1641. Meaning "one who devotes himself solely to attendance on a lady" is from 1817, roughly translating Italian cavaliere-servente. In classical Latin caballus was "work horse, pack horse," sometimes, disdainfully, "hack, nag." "Not a native Lat. word (as the second -a- would show), though the source of the borrowing is uncertain" [Tucker]. Perhaps from some Balkan or Anatolian language, and meaning, originally, "gelding." The same source is thought to have yielded Old Church Slavonic kobyla.