pentagonal

The story

1. Proto-Indo-European*pénkʷe
2. Proto-Hellenic*pénkʷe
3. Ancient Greekπέντε (pénte)
4. Ancient Greekπεντα- (penta-)
5. Proto-Indo-European*ǵónu
6. Proto-Hellenic*gónu
7. Proto-Hellenic*gonwía
8. Ancient Greekγωνία (gōnía)
9. Ancient Greekπεντάγωνον (pentágōnon)
10. Late Latinpentagōnum
11. Middle Frenchpentagone
12. Englishpentagon
13. Proto-Indo-European*h₂el-
14. Proto-Indo-European*h₂élisder.?
15. English todaypentagonal

From pentagon + -al.