1Of, relating to, or originating in the western part of Great Britain, especially Wales or Cornwall; specifically (historical) of or relating to the Brittonic-speaking peoples inhabiting Great Britain, as distinguished from the Anglo-Saxons.
2Of or relating to Ireland, regarded as a western territory of Britain. In later use: of or relating to British rule in Ireland; favouring a close political or cultural connection between Ireland and Great Britain.
Late 18th century; earliest use found in Richard Polwhele (1760–1838), writer. From west + British, originally after West Briton.