From Greek (14th century), from Greek 'zêlos (ζῆλος)' ("ardor, emulation, rivalry, jealousy").
Great energy or enthusiasm in pursuit of a cause or an objective; fervent devotion.
From Late Latin 'zēlus' (zeal, jealousy), from Greek 'zêlos' (ζῆλος, ardor, eager rivalry, emulation, jealousy), possibly from PIE root *yā- (to seek, request) or *gʷelH- (to pierce). In Greek, 'zêlos' was morally neutral — it could mean positive emulation (striving to match a hero) or negative jealousy (resenting another's success). The same word gave English
'Zeal' and 'jealousy' are etymological twins — both from Greek 'zêlos.' The split happened in transmission: 'zeal' came directly through Latin keeping the positive sense (fervor for a cause), while 'jealousy' passed through Old French 'jalousie,' which kept the negative sense (resentful possessiveness). Same ancient emotion