cad 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.

1630s, "a cadet, student soldier," Scottish form of French cadet (see cadet). From 1730 as "person who runs errands;" meaning "golfer's assistant" is from 1851. A letter from Edinburgh c. 1730 describes the city's extensive and semi-organized "Cawdys, a very useful Black-Guard, who attend ... publick Places to go at Errands; and though they are Wretches, that in Rags lye upon the Stairs and in the Streets at Night, yet are they often considerably trusted .... This Corps has a kind of Captain ... presiding over them, whom they call the Constable of the Cawdys."