Geof·frey of Mon·mouth 
(jĕfrē; mŏnməth) 1100?-1154.
(jĕfrē; mŏnməth) 1100?-1154. English chronicler whose semihistorical Historia Regum Britanniae (c. 1139) popularized Arthurian legend and contains the source material for several of Shakespeare's plays.