underpants

The story

1. Proto-Indo-European*h₁én
2. Proto-Indo-European*h₁entér
3. Proto-Indo-European*h₂én
4. Proto-Indo-European*dʰeh₁-?
5. Proto-Indo-European*(H)n̥dʰí
6. Proto-Indo-European*(H)n̥dʰér
7. Proto-Germanic*under
8. Proto-West Germanic*undar
9. Old Englishunder-
10. Middle Englishunder-
11. Englishunder-
12. Proto-Indo-European*peh₂-
13. Proto-Indo-European*ph₂énts?
14. Proto-Hellenic*pánts
15. Ancient Greekπᾶς (pâs)
16. Ancient Greekἔλεος (éleos)
17. Ancient Greekἐλεέω (eleéō)
18. Ancient Greekἐλεήμων (eleḗmōn)
19. Ancient GreekΠαντελεήμων (Panteleḗmōn)
20. ItalianPantalone
21. Frenchpantalon
22. Englishpantaloon
23. Englishpantaloons
24. Englishpants
25. English todayunderpants

From under- + pants.