The word knight derives from the Old English cniht, "youth" or "hero." In Anglo-Saxon the word cniht meant "boy" or "servant", which is similar to the later German Knecht or "labourer." In the Anglo-Saxon Chronicle a cniht is an armed servant or retainer. So a knight (which in the 14th century was spelt like it is today, but pronounced with a "K") came to mean an armed warrior in the service of a king or lord.
The idea of a knight as a gentle, chivalrous man came later. The word "chivalry" is of French origin, derived from the word chevalier (literally "horseman") which was the French equivalent of knight. Originally this just distinuished the more important soldiers, who rode horses, from the ordinary foot-soldiers, but in the Middle Ages a whole ideal of chivalrous or knightly behaviour grew up involving honour, compassion, courtesy to women etc. It had little to do with the original meaning.
The idea of a knight as a gentle, chivalrous man came later. The word "chivalry" is of French origin, derived from the word chevalier (literally "horseman") which was the French equivalent of knight. Originally this just distinuished the more important soldiers, who rode horses, from the ordinary foot-soldiers, but in the Middle Ages a whole ideal of chivalrous or knightly behaviour grew up involving honour, compassion, courtesy to women etc. It had little to do with the original meaning.