E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Younger branches of noble families are called cadets, because their armorial shields are marked with a difference called a cadency.
Cadet is a student at the Royal Military Academy at Woolwich, the Royal Military College at Sandhurst, or in one of her Majestys training ships, the Excellent and the Britannia. From these places they are sent (after passing certain examinations) into the army as ensigns or second lieutenants, and into the navy as midshipmen. (French, cadet, junior member of a family.)