The story
- Proto-Indo-European*speḱ-
- Proto-Indo-European*spéḱyeti
- Proto-Hellenic*sképt͏̌omai
- Ancient Greekσκέπτομαι (sképtomai)
- Ancient Greekσκεπτικός (skeptikós)
- Latinscepticus
- Proto-Indo-European*h₂el-
- Proto-Indo-European*h₂élisder.?
- English todayskeptical
From Latin scepticus + -al; attested earlier than skeptic. By surface analysis, skeptic + -al.
Etymology from Wiktionary, CC BY-SA 4.0.