zealous

Zealous comes from Middle English zelose, from Latin zēlōsus, formed from zēlus plus the suffix -ōsus. Zēlus itself was borrowed from Ancient Greek ζῆλος (zêlos), meaning 'zeal' or 'jealousy', related to the verb ζηλόω (zēlóō), 'to emulate, to be jealous'.

The Greek root ζῆλος carried both 'zeal' and 'jealousy' as overlapping senses, and that double meaning traveled through Latin into English, where the two ideas eventually split into separate words.

Zealous is a doublet of jealous — both descend from the same Latin zēlōsus, one keeping the sense of eager devotion, the other the sense of guarded envy.