From Middle English ernest, eornest, from Old English eornest, eornost, eornust, from Proto-Germanic *ernustuz ("earnest, strength, solidity, struggle, fight"), a derivative of Proto-Germanic *arniz ("efficient, capable, diligent, sure"), from Proto-Indo-European *er- ("to cause to move, arouse, increase"). Cognate with West Frisian earnst ("earnest, seriousness"), Dutch ernst ("seriousness, gravity, earnest"), German Ernst ("seriousness, earnestness, zeal, vigour"), Icelandic ern ("brisk, vigorous"), Gothic 𐌰𐍂𐌽𐌹𐌱𐌰 ("secure, certain, sure").
The adjective is from Middle English eornest, from Old English eornoste ("earnest, zealous, serious"), from the noun. Cognate with North Frisian ernste ("earnest"), Middle Low German ernest, ernst, German ernst ("serious, earnest").
Of uncertain origin; apparently related to erres. Compare also arles.