What is the origin of the naval term 'boatswain' or 'bosun'?

The boatswain (pronounced bosun) had very specific duties but the name is quite ordinary. 'Swain' means young man and has been in existence in Old English and Old Norse for many years. Sometime in the last couple of hundred years it has become virtually extinct.

BOATSWAIN (pronounced 'bo'sun {';} derived from 'boat' and 'swain,' a servant), the warrant officer of the navy who in sailing-ships had particular charge of the boats, sails, rigging, colours, anchors and cordage. He superintended the rigging of the ship in dock, and it was his duty to summon the cI:ew to work by a whistle. The office still remains, though with functions modified by the introduction of steam. In a merchant ship the boatswain is the foreman of the crew and is sometimes also third or fourth mate.

https://en.wikipedia.org/wiki/Boatswain