underfloor

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*pleth₂-?
13. Proto-Indo-European*pel-?
14. Proto-Indo-European*pleh₂-
15. Proto-Indo-European*pleh₂ros
16. Proto-Germanic*flōraz
17. Proto-Germanic*flōruz
18. Proto-West Germanic*flōr
19. Old Englishflōr
20. Middle Englishflor
21. Englishfloor
22. English todayunderfloor

From under- + floor.