An Etymological Dictionary of the German Language/Annotated/ungestüm
ungestüm, adjective, ‘blustering, impetuous,’ from Middle High German ungestüeme, Old High German ungistuomi (unstuom), adjective, ‘stormy, impetuous’; the unnegatived form of the adjective was extremely rare in Old High German and Middle High German. Based on a verbal root stam, as in Middle High German stęmen (Modern High German stemmen, from *stamjan), ‘to check, restrain,’ which appears also in Modern High German stammeln. —