zeal n.

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."

early 14c., "member of a militant 1st century Jewish sect which fiercely resisted the Romans in Palestine," from Late Latin zelotes, from Greek zēlōtēs "one who is a zealous follower," from zēloōn "to be zealous," from zēlos "zeal" (see zeal). Extended sense of "a fanatical enthusiast" first recorded 1630s (earlier in this sense was zelator, mid-15c.).

"full of zeal" (in the service of a person or cause), 1520s, from Medieval Latin zelosus "full of zeal" (source of Italian zeloso, Spanish celoso), from zelus (see zeal). The sense "fervent, inspired" was earlier in English in jealous (late 14c.), which is the same word but come up through French. Related: Zealously, zealousness.