(am vro) vreiddin c. 1223, Brethyn 1292, 1486, Breiddin 13g., (O) Vreitin c.1160-1 (c.1300), dygen ureitin 1157-60 (c.1300), Brethin 1363. Breythin 1382, le Breithyn 1404, Forest of Brethen 1540-1, (am) fro vreiddin c.1223 (c.1631), (dor) Dygen (c.1157-60 C.1300), y dygen c.1000 (c1400)

Uncertain, though the second element must be Welsh din, 'fort, fortress'. The spellings with Brei-, Vrei-, etc appear to rule out Welsh bre, 'hill' and 'elevated', and tend to favour Welsh braidd, 'hardly, scarcely' etc. Taken in isolation, that may seem an unlikely choice but it is certainly used in combination with other words and elements such as pen, 'head', in breidd ben, 'extremity, end' and cwr, 'corner, part that tapers or ends in a point, in breiddgwr, 'extremity, far end' (see GPC /Universty of Wales Dictionary. The area around Y Breiddin is recorded in some early sources as Y Dygen but its meaning is uncertain. Additional information will be found in the book