oxyhydrogen

The story

1. Proto-Indo-European*h₂eḱ-der.?
2. Ancient Greekὀξῠ́ς (oxŭ́s)
3. Englishoxy-
4. Proto-Indo-European*wed-
5. Proto-Indo-European*wódr̥
6. Proto-Hellenic*údōr
7. Ancient Greekῡ̆̔́δωρ (hū̆́dōr)
8. Ancient Greekὑδρο- (hudro-)
9. Frenchhydro-
10. Proto-Indo-European*ǵenh₁-
11. Proto-Indo-European*ǵénh₁os
12. Proto-Hellenic*génos
13. Ancient Greekγένος (génos)
14. Frenchhydrogène
15. Englishhydrogen
16. English todayoxyhydrogen

From oxy- + hydrogen.