An Etymological Dictionary of the German Language/Annotated/heftig
heftig, adjective, ‘vehement, violent, impetuous,’ from Middle High German hęftec, adjective, ‘remaining firm, persistent,’ then ‘earnest, important, strong.’ It seems to be based upon a blending of two words originally quite distinct, for Modern High German heftig, ‘vehemens,’ is late Old High German heiftig, Middle High German heifte, adverb heifteclîchen, with which Gothic haifsts, Old French haste, as well as Modern High German Hast, are connected.