hydropower

The story

1. Proto-Indo-European*wed-
2. Proto-Indo-European*wódr̥
3. Proto-Hellenic*údōr
4. Ancient Greekῡ̆̔́δωρ (hū̆́dōr)
5. Ancient Greekῠ̔δρο- (hŭdro-)
6. Englishhydro-
7. Proto-Indo-European*pótis
8. Proto-Italic*potis
9. Proto-Indo-European*h₁es-
10. Proto-Indo-European*h₁ésti
11. Proto-Indo-European*bʰuH-
12. Proto-Indo-European*bʰúHt
13. Proto-Italic*som
14. Proto-Italic*possom
15. Latinposse
16. Vulgar Latinpotēre
17. Old Frenchpooir
18. Anglo-Normanpoer
19. Middle Englishpower
20. Englishpower
21. English todayhydropower

From hydro- + power.