zelo-, zel-; zeal-
(Greek > Latin: ardor, fervor; jealousy, jealous)
Etymologically, jealousy and zeal came from the same source. Both originally came from Greek zelos. This passed into post-classical Latin as zelus, which later produced the adjective zelosus. Old French incorporated this as gelos or jelous and passed it on to English.
The Greek word denoted "jealousy" and "fervor, enthusiasm", and it is this strand of meaning that has come down to us as jealous. Jalousie was the French equivalent of jealousy. Most of the words that became distinctive terms for "jealousy" were originally used in a good sense of "zeal" and "emulation".