An Etymological Dictionary of the German Language/Annotated/weissagen
weissagen, verb, ‘to foretell, prophesy, predict,’ from the equivalent Middle High German wîssagen, Old High German wîssagôn, weak verb; it is not organically connected with sagen. Based on Old High German wîȥȥago (wîȥago), ‘prophet,’ a derivative of the Teutonic root wī̆t, ‘to know’ (compare Anglo-Saxon wîtga, ‘prophet’). This form was corrupted to wîssagôn in the Old High German period by connecting it with wîs, ‘wise,’ and sago, ‘speaker,’ or rather with Old High German forasago (Old Low German wâr-sago), ‘prophet.’ Old High German wîȥȥago is properly a substantive formed from the Teutonic adjective *wîtag, ‘knowing, intelligible, wise’ Anglo-Saxon (wîtig).