An Etymological Dictionary of the German Language/Geist
Geist, m., ‘spirit, genius, spectre,’ from MidHG. and OHG. geist, m., ‘spirit (in contrast to body), supernatural being’; corresponding to OSax. gêst, Du. geist, AS. gâst (gœ̂st), E. ghost; common to Teut. in the same sense, but in Goth. ahma (see achten). The prim. meaning of the word (‘agitation’?) is not quite certain; yet Ic. geisa, ‘to rage’ (of fire, passion), and Goth. us-gaisjan, ‘to enrage,’ seem to be allied. Respecting the dental suffix of the Teut. Geist (pre-Teut. ghaisdos), note the Sans. root hîḍ (from hizd), ‘to get angry,’ hêḍas, n., ‘anger,’ to which E. aghast also corresponds.