unzealous

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Germanic*un-
4. Proto-West Germanic*un-
5. Old Englishun-
6. Middle Englishun-
7. Englishun-
8. Proto-Indo-European*yeh₂-
9. Proto-Indo-European*yéh₂los
10. Ancient Greekζῆλος (zêlos)
11. Latinzēlus
12. Proto-Indo-European*h₃ed-
13. Proto-Indo-European*h₃édosder.?
14. Latinzēlōsus
15. Middle Englishzelose
16. Englishzealous
17. English todayunzealous

From un- + zealous.