earnest: [OE] Earnest was originally a much more red-blooded word than it is today. It comes ultimately from a Germanic base *ern- which denoted ‘vigour’ or ‘briskness’. To this was added the noun suffix – ost (earnest was originally a noun), giving Old English eornost, which appears at first to have meant ‘intense passion’, and particularly ‘zeal in battle’. However, by the end of the Old English period there is already evidence of a semantic toning down from ‘intensity of feeling’ to ‘seriousness of feeling’ (as opposed to ‘frivolity’), a process which has culminated in modern English connotations of ‘over-seriousness’.
earnest (adj.1)
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- (cognates: 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.
earnest (adj.2)
"portion of something given or done in advance as a pledge," early 15c., with excrescent -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.