An Etymological Dictionary of the German Language/hart
hart, adj. ‘hard, stiff, severe, stern, difficult, hard by,’ from MidHG. hęrte, hart, adj. (harte, adv.; comp. fast, adv., allied to fest, schon to schön, &c.), ‘hard, firm, difficult, painful,’ OHG. hęrti, harti, hart, adj. (harto, adv.), ‘hard’; comp. AS. heard, ‘hard, strong, brave,’ E. hard (hardy is probably derived directly from Rom. — Fr. hardi, which, however, is a derivative of G. hart), Goth. hardus, adj., ‘hardy, severe.’ A common Teut. adj. from pre-Teut. kartús; comp. Gr. κρατύς, ‘strong, powerful, potentate,’ καρτερός, κρατερύς, ‘strong, staunch, mighty, violent,’ adv., κάρτα, ‘very strongly’ (OHG. harto, adv., ‘very, extremely’); allied perhaps to Sans. krátu-s, m., ‘force, strength’ (root kar, ‘to do, make’), or however to Lith. kartùs, ‘bitter’ (root kṛt, ‘to cut, split’). Others compare Sans. çárdha-s, ‘bold, strong,’ to the Teut. adj.