cadetyoudaoicibaDictYouDict[cadet 词源字典]
cadet: [17] Etymologically, a cadet is a ‘little head’. Its original meaning in English was ‘younger son or brother’, and it came from French cadet, an alteration of a Gascon dialect term capdet ‘chief’. This in turn derived from Vulgar Latin *capitellus ‘little head’, a diminutive form of Latin caput ‘head’ (from which English also gets captain and chief).

The reason for its apparently rather strange change in meaning from ‘chief’ to ‘younger son’ seems to be that the younger sons of Gascon families were in former times sent to the French court to fulfil the role of officers. When English borrowed French cadet, it did so not only in a form that retained the original spelling, but also as caddie or cadee, which originally meant ‘young officer’.

The Scottish version, caddie, gradually developed in meaning over the centuries through ‘person who runs errands’ to, in the 19th century, ‘golfer’s assistant’. Cad, originally ‘unskilled assistant’ [18], is an abbreviation of caddie or cadee.
=> captain, chief[cadet etymology, cadet origin, 英语词源]
cadet (n.)youdaoicibaDictYouDict
c. 1610, "younger son or brother," from French cadet "military student officer," noun use of adjective, "younger" (15c.), from Gascon capdet "captain, chief, youth of a noble family," from Late Latin capitellum, literally "little chief," hence, "inferior head of a family," diminutive of Latin caput "head" (see capitulum). "The eldest son being regarded as the first head of the family, the second son the cadet, or little head" [Kitchin].

Apparently younger sons from Gascon noble families 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.