Brythonic Syllabification: (Bry·thon·ic)Pronunciation: /briˈTHänik/ (also Brittonic /briˈtänik/)Definition of Brythonic adjective denoting, relating to, or belonging to the southern group of Celtic languages, consisting of Welsh, Cornish, and Breton. Compare with Goidelic.Also called P-Celtic. noun the Brythonic languages collectively. Origin:from Welsh Brython 'Britons' + -icBrythonic in other Oxford dictionaries Definition of Brythonic in the British & World English dictionary