An Etymological Dictionary of the German Language/Annotated/Segen
Segen, masculine, ‘blessing, bliss’, enchantment,’ from Middle High German sëgen, Old High German sëgan, neuter, ‘sign of the cross, blessing resulting from it, magic spell’; borrowed on the introduction of Christianity (see Kreuz, Altar, and Priester) from Latin sîgnum; so too Old High German sëganôn, ‘to bless,’ Old Saxon sëgnôn, ‘to bless,’ literally ‘to make the sign of the cross,’ from Latin sîgnâre. Anglo-Saxon sëgen, ‘banner, military emblem,’ must have been borrowed at an earlier period from Latin sîgnum; with the ë of the Teutonic words compare the Old Irish loan-word sén, as well as Italian segno (French enseigne).