One of the mysteries of the English language finally explained.
Water containing dissolved alum, used for various purposes.
Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. From alum + water, after post-classical Latin aqua aluminosa, aqua aluminis, and aqua aluminata, and also Middle Low German allūnwāter, Middle High German alūnwazzer (German Alaunwasser); compare also early uses of alum and aluminous.
alum water/ˈaləm ˌwɔːtə/
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