jealously adv.

c. 1200, gelus, later jelus, "possessive and suspicious," originally in the context of sexuality or romance (in any context from late 14c.), from Old French jalos/gelos "keen, zealous; avaricious; jealous" (12c., Modern French jaloux), from Late Latin zelosus, from zelus "zeal," from Greek zēlos, which sometimes meant "jealousy," but more often was used in a good sense ("emulation, rivalry, zeal"), from PIE root *ya- "to seek, request, desire" (see zeal). In biblical language (early 13c.) "tolerating no unfaithfulness." Also in Middle English sometimes in the more positive sense, "fond, amorous, ardent" (c. 1300) and in the senses that now go with zealous, which is a later borrowing of the same word, from Latin.

Most of the words for 'envy' ... had from the outset a hostile force, based on 'look at' (with malice), 'not love,' etc. Conversely, most of those which became distinctive terms for 'jealousy' were originally used also in a good sense, 'zeal, emulation.' [Buck, pp.1138-9]

Among the ways to express "jealous" in other tongues are Swedish svartsjuka, literally "black-sick," from phrase bara svarta strumpor "wear black stockings," also "be jealous." Danish skinsyg "jealous," literally "skin-sick," is from skind "hide, skin" said to be explained by Swedish dialectal expression fa skinn "receive a refusal in courtship."

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.