Save Gender Masculine Usage Welsh Mythology Pronounced Pron. [sɛi̯θ.ˈɛn.ɪn] [key·simplify] Other Forms FormsSeithennin Meaning & History Etymology unknown.Seithenyn (sometimes spelt Seithennin), sometimes known as Seithenyn of the Feeble Mind, is a figure from Welsh legend, apparently contemporary with King Gwyddno Garanhir.