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 more or less resembling milk had been similarly named (for example milk of almond) since late 14c.
"giving milk, having milk," late 13c., milche, melch, from Old English -milce "milking" (Anglian -melce, West Saxon -mielce), from Proto-Germanic *melik- "milk," from PIE root *melg- "to rub off; to milk." A variant form of milk. Since 18c. applied only to domestic animals and chiefly to cows. Milch-cow is from early 15c.
"make known by open declaration, publish, announce" (a decree, news, etc.), 1520s, from Latin promulgatus, past participle of promulgare "make publicly known, propose openly, publish," probably from pro "forth" (see pro-) + mulgere "to milk" (see milk (n.)), used metaphorically for "cause to emerge." In that case the word is "a picturesque farmers' term used originally of squeezing the milk from the udder" [L.R. Palmer, "The Latin Language"]. Related: Promulgated; promulgating. The earlier verb in English was promulge (late 15c.).
Proto-Indo-European root meaning "to rub off," also "to stroke; to milk," in reference to the hand motion involved in milking an animal. Compare *g(a)lag-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit marjati "wipes off;" Greek amelgein, Latin mulgere, Old Church Slavonic mlesti, Lithuanian melžti "to milk;" Old Irish melg "milk."