English

Adjective

Brittonic ‎(comparative more Brittonic, superlative most Brittonic)

1. Brythonic; pertaining to the Celtic people inhabiting Britain before the Roman conquest, and to their language. [from 19th c.]
   • 2014, Ellie Mae O'Hagen, The Guardian, 23 Jul 2014:

     The Welsh language is not a backwards, insignificant thing; it is a fundamental part of Britain’s collective history. I find it absurd that so few English people realise it is still spoken in families and communities across Wales, as part of a Brittonic culture which has survived through the ages.

Proper noun

Brittonic

1. The group of P-Celtic languages. [from 20th c.]