"George's name for him, 'The Buccaneer' - about which there was something romantic"
Public DomainBuccaneer

Buccaneers were pirates, largely associated with the Caribbean.  Because they attacked primarily Spanish shipping, they were seen by the English as romantic and brave rather than merely criminal and murderous.

Bosinney also reminded Swithin of Alexander Dumas's fictional swashbuckling hero, the Count of Monte Cristo.

Galsworthy's biographer, Catherine Dupré, suggests that Bosinney was his fictional self.