ζῆλος

Greek zēlos (ζῆλος) meant 'zeal,' 'eager rivalry,' and 'emulation.' It was a compliment. Aristotle distinguished zēlos from phthonos (envy) in his Rhetoric, written around 350 BCE — zēlos was the desire to match someone's excellence, while phthonos was resentment of it. A person with zēlos wanted to be as good as you. A person with phthonos wanted you to fail.

The Septuagint — the Greek translation of the Hebrew Bible, completed around 250 BCE in Alexandria — used zēlos to translate the Hebrew qin'ah, which described God's jealousy over Israel. This was the turning point. Zēlos absorbed the possessive, suspicious meaning of qin'ah. By the time Latin adopted it as zelus, the word carried both senses: righteous passion and suspicious possessiveness.

Old French compressed both meanings into jalous, which entered Middle English as jelous around 1200 CE. English then split the word in two. 'Zeal' and 'zealot' kept the original Greek sense of passionate commitment. 'Jealous' and 'jealousy' took the darker meaning — the fear that someone will take what is yours. One root, two words, opposite connotations.

The split happened gradually. Shakespeare used 'jealous' in both senses — Othello's jealousy is possessive suspicion, but 'jealous in honour' in Henry V means zealously protective. By the 1700s, the separation was complete. Zeal was admirable. Jealousy was not.