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 woman's servant or swain.
The name of Byron's favourite dog, buried in Newstead Abbey garden.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894