Brythonic

[bri-thon-ik] /brɪˈθɒn ɪk/
adjective
1.
of or belonging to P-Celtic.
noun
2.
P-Celtic, especially that part either spoken in Britain, as Welsh and Cornish, or descended from the P-Celtic speech of Britain, as Breton.
Also, Britannic, Brittonic.
Origin
1880-85; Brython + -ic
British Dictionary definitions for Brythonic

Brythonic

/brɪˈθɒnɪk/
noun
1.
the S group of Celtic languages, consisting of Welsh, Cornish, and Breton
adjective
2.
of, relating to, or characteristic of this group of languages
Popular name Brittonic
Word Origin and History for Brythonic
adj.

"of the Britons, Welsh," 1884, from Welsh Brython, cognate with Latin Britto (see Briton). Introduced by Welsh Celtic scholar Professor John Rhys (1840-1915) to avoid the confusion of using Briton/British with reference to ancient peoples, religions, and languages.