An Etymological Dictionary of the German Language/Annotated/Ernst
Ernst, masculine, ‘earnestness, seriousness, gravity,’ from Middle High German ërnest, masculine, Old High German ërnust, neuter, feminine, ‘contest, earnest, decision of character’; corresponding to Dutch ernst, Anglo-Saxon eornost, ‘duel, earnest,’ English earnest; the suffix -n-ust as in Dienst; see also Angst. Akin also to Old Icelandic orrosta, ‘battle’; the stem er (erz?, ers?) is not found elsewhere with a similar meaning; the evolution in meaning resembles that of Kampf, Krieg. The cognates in other Aryan languages are uncertain. — The adjective ernst, simply Modern High German, is represented by ërnesthaft in Middle High German and by ërnusthaft and ërnustlîch in Old High German