The story
- Proto-Indo-European*swéḱs
- Proto-Hellenic*hwéks
- Ancient Greekἕξ (héx)
- Ancient Greekἑξα- (hexa-)
- Latinhexa-
- Englishhexa-
- Frenchméthylène
- Englishmethylene
- New Latinammōnium
- Englishammonium
- Englishonium
- English todayhexamethonium
From hexa- + meth(ylene) + -onium (“quaternary ammonium compound”).
Etymology from Wiktionary, CC BY-SA 4.0.