Why is the ninth month called September?
c.1200, gelus, later jelus (early 14c.), "possessive and suspicious," originally in the context of sexuality or romance; in general use late 14c.; also in a more positive sense, "fond, amorous, ardent," from c.1300, from Old French jalos "keen, zealous; avaricious; jealous" (12c., Modern French jaloux), from Late Latin zelosus, from zelus "zeal," from Greek zelos, sometimes "jealousy," but more often in a good sense ("emulation, rivalry, zeal"). See zeal. In biblical language (early 13c.) "tolerating no unfaithfulness."
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 this 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."