An Etymological Dictionary of the German Language/Segen
Segen, m., ‘blessing, bliss’, enchantment,’ from MidHG. sëgen, OHG. sëgan, n., ‘sign of the cross, blessing resulting from it, magic spell’; borrowed on the introduction of Christianity (see Kreuz, Altar, and Priester) from Lat. sîgnum; so too OHG. sëganôn, ‘to bless,’ OSax. sëgnôn, ‘to bless,’ lit. ‘to make the sign of the cross,’ from Lat. sîgnâre. AS. sëgen, ‘banner, military emblem,’ must have been borrowed at an earlier period from Lat. sîgnum; with the ë of the Teut. words comp. the OIr. loan-word sén, as well as Ital. segno (Fr. enseigne).