biarsenical

The story

1. Proto-Indo-European*dwóh₁
2. Proto-Indo-European*dwi-
3. Proto-Italic*dwi-
4. Latinbi-
5. Englishbi-
6. Proto-Indo-European*ǵʰelh₃-
7. Proto-Indo-European*ǵʰl̥h₃enyom
8. Proto-Indo-Iranian*ȷ́ʰr̥Hanyam
9. Proto-Iranian*jr̥Hanyam
10. Old Median*zaranyam
11. Old Median*kā
12. Old Median*zaraniyakā
13. Middle Persian*zarnīk
14. Classical Syriacܙܪܢܝܟܐ (zarnīḵā)
15. Arabicالزَرْنِيخ (az-zarnīḵ)
16. Medieval Greekἀρσενικόν (arsenikón)
17. Latinarsenicum
18. Proto-Indo-European*h₂el-
19. Proto-Indo-European*h₂élisder.?
20. Englisharsenical
21. English todaybiarsenical

From bi- + arsenical.