An Etymological Dictionary of the German Language/weissagen
weissagen, vb., ‘to foretell, prophesy, predict,’ from the equiv. MidHG. wîssagen, OHG. wîssagôn, wk. vb.; it is not organically connected with sagen. Based on OHG. wîȥȥago (wîȥago), ‘prophet,’ a derivative of the Teut. root wī̆t, ‘to know’ (comp. AS. wîtga, ‘prophet’). This form was corrupted to wîssagôn in the OHG. period by connecting it with wîs, ‘wise,’ and sago, ‘speaker,’ or rather with OHG. forasago (OLG. wâr-sago), ‘prophet.’ OHG. wîȥȥago is properly a subst. formed from the Teut. adj. *wîtag, ‘knowing, intelligible, wise’ AS. (wîtig).