Origins
The word 'jealous' entered English around 1200 from Old French 'jalous' (also spelled 'gelos'), from Medieval Latin 'zelōsus' (full of zeal), from Late Latin 'zēlus,' borrowed from Greek 'ζῆλος' (zēlos). The Greek word had a wide semantic range: 'zeal,' 'ardour,' 'emulation,' 'rivalry,' and 'jealousy.' The sound change from initial /z/ to /dʒ/ (the 'j' sound) occurred in Old French through regular phonological development.
English 'jealous' and English 'zealous' are doublets — two words descended from the same source that entered the language by different paths. 'Zealous' came more directly through Latin, preserving the initial 'z.' 'Jealous' travelled through Old French, where the sound shifted. The semantic split is striking: 'zealous' retained the positive sense of passionate enthusiasm, while 'jealous' absorbed the negative sense of suspicious, envious rivalry. Both meanings were present in the Greek original, but English divided them between two words.
The Greek noun 'ζῆλος' (zēlos) was itself a neutral term that could describe both admirable and destructive passion. Aristotle distinguished between 'zēlos' (emulation — seeing another's excellence and being inspired to match it, a positive response) and 'phthonos' (envy — seeing another's good fortune and wanting to destroy it, a negative response). In the Septuagint and New Testament, 'zēlos' was used for God's jealousy — His fierce, protective love for His people — giving the word a theological dimension that shaped both 'jealous' and 'zealous' in English.
Development
The biblical phrase 'a jealous God' (Exodus 20:5) uses 'jealous' in this older, non-pejorative sense: God is fiercely protective of His relationship with Israel, as a spouse might be vigilant over the exclusivity of a marriage. This sense — vigilant, fiercely guarding what is one's own — survives in phrases like 'jealous of one's reputation' and 'jealously guarded secrets.'
In modern usage, 'jealous' is most commonly associated with romantic jealousy — the fear that a partner's affections are being diverted to a rival. Shakespeare explored jealousy with devastating depth: Othello, manipulated by Iago into suspecting Desdemona's faithfulness, is told that jealousy is 'the green-eyed monster which doth mock the meat it feeds on.' The association of jealousy with green (the 'green-eyed monster,' 'green with envy') may derive from ancient Greek medical theory, which associated green-tinged bile with bitter, envious dispositions.
Italian 'geloso' and its architectural derivative 'gelosia' (a type of latticed window screen) preserve an unexpected sense: the screen allowed women in Mediterranean households to look out without being seen — a device associated with jealous husbands guarding their wives from the gaze of others. The English word 'jalousie' (a louvred window) comes from this same connection.