An Etymological Dictionary of the German Language/Annotated/hart
hart, adjective ‘hard, stiff, severe, stern, difficult, hard by,’ from Middle High German hęrte, hart, adjective (harte, adverb; compare fast, adverb, allied to fest, schon to schön, &c.), ‘hard, firm, difficult, painful,’ Old High German hęrti, harti, hart, adjective (harto, adverb), ‘hard’; compare Anglo-Saxon heard, ‘hard, strong, brave,’ English hard (hardy is probably derived directly from Romance — French hardi, which, however, is a derivative of German hart), Gothic hardus, adjective, ‘hardy, severe.’ A common Teutonic adjective from pre-Teutonic kartús; compare Greek κρατύς, ‘strong, powerful, potentate,’ καρτερός, κρατερύς, ‘strong, staunch, mighty, violent,’ adverb, κάρτα, ‘very strongly’ (Old High German harto, adverb, ‘very, extremely’); allied perhaps to Sanscrit krátu-s, masculine, ‘force, strength’ (root kar, ‘to do, make’), or however to Lithuanian kartùs, ‘bitter’ (root kṛt, ‘to cut, split’). Others compare Sanscrit çárdha-s, ‘bold, strong,’ to the Teutonic adjective.