The word 'zeal' has a passionate history, traveling from ancient Greek into the heart of English religious and secular fervor. It entered Middle English via ecclesiastical Latin 'zēlus', which borrowed the Greek 'zēlos', a term signifying ardent rivalry, fervent pursuit, or jealous emulation. This Greek root itself may be distantly connected to Proto-Indo-European ideas of fermentation or boiling, a vivid metaphor for intense inner agitation. Over time, the English word shed its primary associations with jealousy and rivalry, refining its meaning to denote intense, enthusiastic devotion to a cause, person, or ideal.

This word is a simplex loanword. It entered English as a whole unit from Late Latin 'zēlus', which came from Greek 'zēlos' (ardour, emulation, zeal). The Greek word is of uncertain deeper origin but may be related to concepts of boiling or fermenting, metaphorically extended to fervor.