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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*(s)kelH-
13. Proto-Indo-European*(s)kel(H)-p-
14. Proto-Germanic*skelfō
15. Proto-West Germanic*skelfu:
16. Proto-West Germanic*skilfijā
17. Old Englishsċylfe?
18. Middle Englishschelfe
19. Englishshelf
20. English todayundershelf

From under- + shelf.