From Middle English zel, from Old French zel or Medieval Latin zelus, from Late Latin zelus (fervor, jealousy), from Greek zēlos (ardor, rivalry, jealousy) — from a root meaning 'to boil, to seethe'. In ancient Greek, zēlos encompassed both passionate devotion and jealous rivalry. In Christian Latin, zelus became associated primarily with devout religious fervor. English borrowed the word primarily in the sense of ardent enthusiasm and devotion, which it has retained.