caddie n.

c. 1610, "younger son or brother;" 1650s, "gentleman entering the military as a profession;" from French cadet "military student officer," noun use of adjective, "younger" (15c.), from Gascon capdet "captain, chief, youth of a noble family," from Medieval Latin capitellum, "little chief," literally "little head" (hence, "inferior head of a family"), diminutive of Latin caput "head" (from PIE root *kaput- "head").

"The eldest son being regarded as the first head of the family, the second son the cadet, or little head" [Kitchin]. Younger sons from noble families were sent to French court to serve as officers, without rising through the ranks or attending military school, after being attached to a corps without pay and enjoying certain privileges. This gave the word its military meaning "accepted candidate for a commission who is undergoing training to become an officer." Meaning "student at a military college" is from 1775.

Via the Scottish form cadee comes caddie "a messenger boy," especially one who carries clubs for a golfer, and slang cad.

1730, shortening of cadet (q.v.); originally used of servants, then (1831) of town boys by students at Oxford and English public schools (though at Cambridge it meant "snob"), then "townsman" generally. Compare caddie. The meaning "person lacking in finer feelings" is from 1838.

A cad used to be a jumped-up member of the lower classes who was guilty of behaving as if he didn't know that his lowly origin made him unfit for having sexual relationships with well-bred women. [Anthony West, "H.G. Wells: Aspects of a Life," 1984]