The word Caddie derives from the French word 'le cadet', meaning 'the boy' or the youngest of the family. The word 'cadet' appears in English from 1610 and the word 'caddie' or 'cadie' shortly after that in 1634. Until recently, the word 'Garçon!' meaning boy used to be used to summon waiters, but is no longer correct. Adopting French terms was not unusual for the Scots. For example they adopted the term 'Gardez l'eau!' as gardy-loo to warn passers-by when they were throwing dirty water out of their windows into the streets.
This appears to be the origin of the speculative theory that French military 'cadets' carried the clubs for the golfing royalty in France and this practice came to Scotland when Queen Mary Stuart returned in 1561. This is not possible as the French did not play golf, but maile, which only involved one club, though the military 'cadet' has the same origin, as they were often the younger sons of the aristocracy.
A Cady, Caddy, Cadie or Caddie became used for a general-purpose porter or errand boy in Scottish towns in the 18th Century, particularly used for delivering water in the days before modern utilities. They were formed into a society in Edinburgh in 1711, with self imposed rules and published fees much like taxi cabs. The Shorter Oxford Dictionary records this use as a general porter from 1730. Caddies are often mentioned carrying golf clubs, but it was not until 1857 that the Dictionary ascribes the use mainly to those carrying golf clubs. In the early days there were no bags and the clubs were carried in bundle, which can be clearly seen in paintings of the time.
The first named caddie was Andrew Dickson, who would later become a golf clubmaker and who acted as fore-caddie for the Duke of York as a boy in 1681 in the Duke's golf match on Leith Links, as outlined in Fore! In the times of 'featherie' golf balls forecaddies were common as featheries were expensive.
Another well-known early caddie was Daft Willie Gunn at Bruntsfield. Some caddies were older professionals involved in golf, but many were young boys. Willie Gunn was one of the few who was neither, but simply earned his corn carrying clubs.