An Etymological Dictionary of the German Language/Ernst
Ernst, m., ‘earnestness, seriousness, gravity,’ from MidHG. ërnest, m., OHG. ërnust, n., f., ‘contest, earnest, decision of character’; corresponding to Du. ernst, AS. eornost, ‘duel, earnest,’ E. earnest; the suffix -n-ust as in Dienst; see also Angst. Akin also to OIc. 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 adj. ernst, simply ModHG., is represented by ërnesthaft in MidHG. and by ërnusthaft and ërnustlîch in OHG.