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1880, proprietary name for white suspension of magnesium hydroxide in water, taken as an antacid, invented by U.S. chemist Charles Henry Phillips. Herbal or culinary preparations resembling milk had been similarly named (e.g. milk of almond) since early 15c.
milk of magnesia n.
A milky white aqueous suspension of magnesium hydroxide used as an antacid and a laxative.