I was browsing through the OED the other day and came across the entry for cadet. Turns out it originally meant a younger son or brother (particularly the youngest son). It also came to mean “a gentleman who entered the army without a commission, to learn the military profession and find a career for himself (as was regularly done by the younger sons of the French nobility before the Revolution),” and, finally, “a student in a military or naval college,” which is how it’s mainly used today. (I should add that in New Zealand it’s used to refer to a young man learning sheep-farming on a sheep-station. Not quite military but still cool.)
Cadet comes from the French cadet (surprising, I know), which comes from the Provençal word capdet, which itself is from the diminutive of the Latin caput (“head”). So it meant “little chief,” or the “inferior head of a family.”
As for the history of caput, it’s related to the Greek κεϕαλή (also meaning “head”), which is where we get words like hypocephalus (as in the Book of Abraham — “under the head”) and encephalitis (ἐγκέϕαλος means “brain,” with the -itis “disease” suffix).
You also get cool words like bicephalous (“two-headed”), cebocephalic (“monkey-headed”), cephalalgy (fancy word for “headache”), cynocephalus (“one of a fabled race of men with dogs’ heads”), ophiocephale (“serpent-headed”), and pachycephalic (“having a very thick skull,” and yes, it also means “thick-headed” and “stupid”). And pachycephalic may remind you of pachyderm (“thick-skinned”), which we use to refer to animals like elephants, rhinos, and hippos.
Getting back to caput, its Latin relatives include capillaris (“of or pertaining to the hair”), Capitolium (the Roman Capitol), praeceps (“headlong, steep,” whence we get precipice), and biceps (“two-headed” or “divided into two parts”).