trimethylarsine

The story

1. Proto-Indo-European*tréyes
2. Proto-Indo-European*tri-
3. Proto-Italic*tri-
4. Latintri-
5. Englishtri-
6. Proto-Indo-European*médʰu
7. Proto-Hellenic*métʰu
8. Ancient Greekμέθυ (méthu)
9. Proto-Indo-European*swel-der.?
10. Ancient Greekῡ̔́λη (hū́lē)
11. GermanMethy
12. Englishmethyl
13. Englishtrimethyl
14. Proto-Indo-European*ǵʰelh₃-
15. Proto-Indo-European*ǵʰl̥h₃enyom
16. Proto-Indo-Iranian*ȷ́ʰr̥Hanyam
17. Proto-Iranian*jr̥Hanyam
18. Old Median*zaranyam
19. Old Median*kā
20. Old Median*zaraniyakā
21. Middle Persian*zarnīk
22. Classical Syriacܙܪܢܝܟܐ (zarnīḵā)
23. Arabicالزَرْنِيخ (az-zarnīḵ)
24. Medieval Greekἀρσενικόν (arsenikón)
25. Latinarsenicum
26. Middle Frencharsenic
27. Middle Englisharsenik
28. Englisharsenic
29. Englisharsine
30. English todaytrimethylarsine

From trimethyl + arsine.