- cadet c.1610, "younger son or brother," from Fr. cadet "military student officer," also, as an adj., "younger" (15c.), from Gascon capdet "captain, chief, youth of a noble family," from L.L. capitellum, lit. "little chief," hence, "inferior head of a family," dim. of L. caput "head" (see HEAD (Cf. head)). Younger sons from Gascon noble families apparently were sent to French court to serve as officers, which gave the word its military meaning. In English, the meaning "gentleman entering the military as a profession" is from 1650s, and that of "student at a military college" is from 1775.
Etymology dictionary. 2014.