The word knight, from Old English cniht ("boy" or "servant"), is a cognate of the German word Knecht ("servant, bondsman"). This meaning, of unknown origin, is common among West Germanic languages (cf Old Frisian kniucht, Dutch knecht, Danish knægt, Swedish knekt, Norwegian knekt, Middle High German kneht, all meaning "boy, youth, lad", as well as German Knecht "servant, bondsman, vassal"). Middle High German had the phrase guoter kneht, which also meant knight; but this meaning was in decline by about 1200.
The meaning of cniht changed over time from its original meaning of "boy" to "household retainer". Ælfric's homily of St. Swithun describes a mounted retainer as a cniht. While cnihtas might have fought alongside their lords, their role as household servants features more prominently in the Anglo-Saxon texts. In several Anglo-Saxon wills cnihtas are left either money or lands. In his will, King Æthelstan leaves his cniht, Aelfmar, eight hides of land.
A rādcniht, "riding-servant", was a servant delivering messages or patrolling coastlines on horseback.
A narrowing of the generic meaning "servant" to "military follower of a king or other superior" is visible by 1100. The specific military sense of a knight as a mounted warrior in the heavy cavalry emerges only in the Hundred Years' War. The verb "to knight" (to make someone a knight) appears around 1300; and, from the same time, the word "knighthood" shifted from "adolescence" to "rank or dignity of a knight".
An Equestrian (Latin, from eques "horseman", from equus "horse") was a member of the second highest social class in the Roman Republic and early Roman Empire. This class is often translated as "knight"; the medieval knight, however, was called miles in Latin (which in classical Latin meant "soldier", normally infantry).
In the later Roman Empire, the classical Latin word for horse, equus, was replaced in common parlance by the vulgar Latin caballus, sometimes thought to derive from Gaulish caballos. From caballus arose terms in the various Romance languages cognate with the (French-derived) English cavalier: Italian cavaliere, Spanish caballero, French chevalier (whence chivalry), Portuguese cavaleiro, and Romanian cavaler. The Germanic languages have terms cognate with the English rider: German Ritter, and Dutch and Scandinavian ridder. These words are derived from Germanic rīdan, "to ride", in turn derived from the Proto-Indo-European root reidh-.