An Etymological Dictionary of the German Language/heftig

by Friedrich Kluge

heftig, adj., ‘vehement, violent, impetuous,’ from MidHG. hęftec, adj., ‘remaining firm, persistent,’ then ‘earnest, important, strong.’ It seems to be based upon a blending of two words orig. quite distinct, for ModHG. heftig, ‘vehemens,’ is late OHG. heiftig, MidHG. heifte, adv. heifteclîchen, with which Goth. haifsts, OFr. haste, as well as ModHG. Hast, are connected.