unskeptical

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*speḱ-
9. Proto-Indo-European*spéḱyeti
10. Proto-Hellenic*sképt͏̌omai
11. Ancient Greekσκέπτομαι (sképtomai)
12. Ancient Greekσκεπτικός (skeptikós)
13. Latinscepticus
14. Proto-Indo-European*h₂el-
15. Proto-Indo-European*h₂élisder.?
16. Englishskeptical
17. English todayunskeptical

From un- + skeptical.