An Etymological Dictionary of the German Language/Annotated/Geisel
Geisel (1.), masculine and feminine, ‘hostage,’ from Middle High German gisel, Old High German gisal, masculine, neuter, ‘prisoner of war, person held in security’; corresponding to Anglo-Saxon gîsel, Old Icelandic gîsl, masculine. To connect it with Geisel (2.), feminine, as if ‘hostage’ were literally ‘one who is scourged,’ is impossible. It is, probably, most closely allied to the equivalent Old Irish giall (for *gísal).
Geisel (2.) feminine, ‘scourge, whip,’ from the equivalent Middle High German geisel), Old High German geisala, geisla, feminine; akin to Old Icelandic geisl, geisle, masculine, ‘pole used by persons walking in snow-shoes.’ The stem gais- is connected with the Old Teutonic term gaiza-, ‘spear’ (see Ger). Hence ‘pole, staff,’ must be accepted as the primary meaning; the second component is Gothic walus, ‘staff,’ so that Old High German geis-ala stands for *geis-wala, just as Old High German wurzala for Anglo-Saxon wyrt-walu (see under Wurzel).