An Etymological Dictionary of the German Language/Annotated/Ritten
Ritten, masculine, ‘fever,’ from the equivalent Middle High German rite, ritte, Old High German rito, ritto, masculine, for an earlier *hriþjo, ‘fever’; so too Anglo-Saxon hriþa, masculine, ‘fever’; allied to Old High German rîdon, Middle High German rîden, ‘to shiver,’ Anglo-Saxon hriþian, ‘to shiver in a fever,’ Old High German rîdo, ‘shivering,’ Anglo-Saxon hriþ, ‘storm.’ The root hrī̆þ, pre-Teutonic krī̆t, ‘to move wildly,’ appears also in Old Irish crith, ‘shivering.’