ca·det / kəˈdet/ • n. 1. a young trainee in the armed services or police force: an air force cadet. ∎  a student in training at a military school.2. formal or archaic a younger son or daughter. ∎  [usu. as adj.] a junior branch of a family: a cadet branch of the family.DERIVATIVES: ca·det·ship / -ˌship/ n.ORIGIN: early 17th cent. (sense 2): from French, from Gascon dialect capdet, a diminutive based on Latin caput ‘head.’ The notion “little head” or “inferior head” gave rise to that of ‘younger, junior.’

cadet younger son, etc.; gentleman in the army without a commission XVII; junior officer XVIII. — F. cadet, earlier capdet — Gascon dial. capdet :- Rom. *capitellus, dim. of caput head, CHIEF; orig. applied to Gascon officers (younger sons of noble families) at the French court.