Etymology Dictionary

CADET,  a younger son, young military student.  (F.,—Low L.,—L.)   'The cadet of an antient and noble family;' Wodd's Athenæ Oxonienses (R.)   'The cadet of a very ancient family;' Tatler, no. 256 [not 265].—F. cadet, 'a younger brother among gentlemen;' a Poitou word; Cot.   The Prov. form is capdet (Brachet), formed from a Low Lat. capitettum, a neuter form not found, but inferred from the Provençal.   This Low Lat. capitettum would mean lit. 'a little head.'   The eldest son was called caput, the 'head' of the family, the second the capitettum, or 'lesser head.'—Lat. caput, the head, cognate with E. head, q.v.   Der. cad (a slang word, being a mere abbreviation of cadet, like cab from cabriolet ); cadet-ship. [†]

ADDENDA

M. Paul Meyer informs me that capdet is probably a Gascon form, and that it does not represent Low Lat. capitettum, but Low Lat. capitellum, by a habit of Gascon which puts final t for final ll.