Where does “уксусъ” come from?

уксусъ (Old East Slavic) comes from Ancient Greek ὄξος, from Ancient Greek ὀξύς, from Proto-Indo-European h₂eḱ- — sharp.

Ancestry of “уксусъ”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	ὄξος	poor wine; vinegar made therefrom; sour fellow,...
2	Ancient Greek	ὀξύς	sharp, pointed; sharp, keen; quick, hasty, swift
3	Proto-Indo-European	h₂eḱ-	sharp