Wiktionary
captain's servant
n. (context nautical English) (''before 1796'') the rating of a boy entering the Royal Navy at about the age of twelve before becoming a midshipman
Usage examples of "captain's servant".
It had been the cupboard of the Dutch captain's servant, and was a mere cubby-ole.
The son of a man who held one of the oldest earldoms in the kingdom and who had been one of the country's most famous admirals could have expected very rapid promotion if he chose the Navy: captain's servant or midshipman in ships whose captains would ensure he had the finest training.