mother of vinegar
noun
A ropy slime which forms on the surface of alcoholic liquids during acetogenic fermentation and is used to initiate such fermentation in other alcoholic liquids; (also) the organism which produces this slime, which is a bacterium of the genus Acetobacter (formerly thought to be a fungus).
Origin
Early 17th century; earliest use found in Philemon Holland (1552–1637), translator.