Douglas Harper's Etymology Dictionary
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 (for example milk of almond) since early 15c.
Wiktionary
n. A milky-white solution of magnesium hydroxide, used as a saline osmotic laxative and as an antacid.
WordNet
n. purgative consisting of a milky white liquid suspension of magnesium hydroxide; used as a laxative and (in smaller doses) as an antacid [syn: magnesium hydroxide]
Usage examples of "milk of magnesia".
But even if every member of the family had a Slurpee and a personal bottle of Milk of Magnesia for dealing with the aftereffects, that would still leave three cupholders spare.
Sky Captain reached back into the deep drawer and withdrew a bottle: milk of magnesia.
An' no more lovey-kindness than a stick, an' no more gratitude than a glass o' milk of magnesia.
Retiree vagabonds of a certain age, already worried about turning radiuses and tricky angles of approach to their campsite hookups, turned as pale as Milk of Magnesia if they were unfortunate enough to be required to slot-park their humbler Winnebagos and Air-streams in this beast's shadow, and most regarded the leviathan with resentment or paranoid terror.
When I got back to Maggody, I stopped at my apartment for a slug of milk of magnesia, then checked in with Officer Buchanon at the PD.
There is a bottle of Vivarin, a bottle of Serutan (That's 'Nature's' spelled backwards, the ads on Lawrence Welk used to say when Eddie Kaspbrak was but a wee slip of a lad), and two bottles of Phillips Milk of Magnesia - the regular, which tastes like liquid chalk, and the new mint flavor, which tastes like mint-flavored liquid chalk.
Aspirin, tooth powder, milk of magnesia, and small vials of prescription drugs competed for space with jars of foul-smelling powders marked by obscure alchemical symbols.
The lenses of his dark glasses were the blue of old Milk of Magnesia bottles.