zealous adj.

"passionate ardor in pursuit of an objective or course of action," late 14c., from Old French zel (Modern French zèle) and directly from Late Latin zelus "zeal, emulation" (source also of Italian zelo, Spanish celo), a Church word, from Greek zēlos "ardor, eager rivalry, emulation," "a noble passion" [Liddell & Scott], but also "jealousy;" from PIE *ya- "to seek, request, desire." From mid-15c. as "devotion."

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."

also over-zealous, "too zealous, exhibiting an excess of zeal," 1630s, from over- + zealous. Related: Overzealously; overzealousness.