The story
- Proto-Indo-European · derived from*duwó
- Ancient Greek · borrowed fromδυάς
- English todaydyad
From Ancient Greek δυάς (duás), δυάδ- (duád-) from δύο (dúo, “two”), from Proto-Indo-European *duwó, *duwéh₃ (*dwóh₁). The mathematics sense was coined by American scientist Josiah Willard Gibbs in 1884 in the second half of his book Elements of Vector Analysis.
Etymology from Wiktionary, CC BY-SA 4.0.