from Old English eornoste (adj.) "zealous, serious," or from Old English noun eornost "seriousness, serious intent" (surviving only in the phrase in earnest), from Proto-Germanic *er-n-os-ti- (source also of Old Saxon ernust, Old Frisian ernst, Old High German arnust "seriousness, firmness, struggle," German Ernst "seriousness;" Gothic arniba "safely, securely;" Old Norse ern "able, vigorous," jarna "fight, combat"), perhaps from PIE root *er- (1) "to move, set in motion." The proper name Ernest (literally "resolute") is from the same root. Related: Earnestness.
"portion of something given or done in advance as a pledge," early 15c., with unetymological -t- (perhaps from influence of the other earnest), from Middle English ernes (c. 1200), "a pledge or promise;" often "a foretaste of what is to follow;" also (early 13c.) "sum of money as a pledge to secure a purchase or bind a bargain (earnest-money); from Old French erres and directly from Latin arra, probably from Phoenician or another Semitic language (compare Hebrew 'eravon "a pledge"). Sometimes in Middle English as erness, suggesting it was perceived as er "early" + -ness.