E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The officer who has charge of the boats, sails, rigging, anchors, cordage, cables, and colours. Swain is the Saxon swein (a boy, servant), Swedish sven. Hence, a shepherd is a swain, and a sweetheart is a womans servant or swain.
Boatswain. The name of Byrons favourite dog, buried in Newstead Abbey garden.