"serious or grave in speech or action," early 14c., ernest, 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.
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.