hydromagnetic

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. Old Frenchmagnete
8. Ancient GreekΜάγνης (Mágnēs)
9. Ancient Greekμᾰ́γνης (mắgnēs)
10. Latinmagnēs
11. Middle Englishmagnete
12. Englishmagnet
13. Englishmagnetic
14. English todayhydromagnetic

From hydro- + magnetic.