Etymology
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milky (adj.)
late 14c., "milk-like in color or consistency," from milk (n.) + -y (2). Related: Milkily; milkiness.
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Milky Way (n.)

"the galaxy as seen in the night sky," late 14c., loan-translation of Latin via lactea; see galaxy. Formerly in Middle English also Milken-Way and Milky Cercle. The ancients speculated on what it was; some guessed it was a vast assemblage of stars (Democrates, Pythagoras, even Ovid); the question was settled when Galileo, using his telescope, reported that the whole of it was resolvable into stars. Old native names for it include Jacob's Ladder, the Way to St. James's, and Watling Street (late 14c.).

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lactescence (n.)
"milky appearance," 1680s, from lactescent "becoming milky" (1660s), from Latin lactescentem (nominative lactescens), present participle of lactescere, inchoative of lactere "to be milky," from lac "milk" (from PIE root *g(a)lag- "milk").
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galacto- 
before vowels galact-, word-forming element meaning "milk, milky," from Greek gala (stem galakt-; see galaxy).
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galaxy (n.)

late 14c., from French galaxie or directly from Late Latin galaxias "the Milky Way" as a feature in the night sky (in classical Latin via lactea or circulus lacteus), from Greek galaxias (adj.), in galaxias kyklos, literally "milky circle," from gala (genitive galaktos) "milk" (from PIE root *g(a)lag- "milk").

The technical astronomical sense in reference to the discrete stellar aggregate including the sun and all visible stars emerged by 1848. Figurative sense of "brilliant assembly of persons" is from 1580s. Milky Way is a translation of Latin via lactea.

See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. [Chaucer, "House of Fame"]

Originally ours was the only one known. Astronomers began to speculate by mid-19c. that some of the spiral nebulae they could see in telescopes were actually immense and immensely distant structures the size and shape of the Milky Way. But the matter was not settled in the affirmative until the 1920s.

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opalescence (n.)

"iridescence like that of an opal, a play of colors milky rather than brilliant," 1792; see opalescent + -ence. Perhaps via French opalescence.

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chyle (n.)

"milky fluid formed during the process of digestion," 1540s, from Late Latin chylus "the extracted juice of a plant," from Greek khylos "juice" (of plants, animals, etc.), from stem of khein "to pour, gush forth," from PIE *ghus-mo-, from root *gheu- "to pour, pour a libation." Compare also chyme.

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galactic (adj.)
1839, "of the Milky Way, of the bright band of stars around the night sky," from Late Latin galacticus, from galaxias (see galaxy). In modern scientific sense "pertaining to (our) galaxy," from 1849. From 1844 as "of or pertaining to milk."
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Minnesota 

originally the name of the river, from Dakota (Siouan) mnisota, literally "cloudy water, milky water," from mni "river, stream" + sota "slightly clouded." As the name of a U.S. territory from 1849 (admitted as a state 1858). Related: Minnesotan (by 1867).

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lacteal (adj.)
1650s, "pertaining to milk," earlier "milk-white" (1630s), from Latin lacteus "milky" (from lac "milk," from PIE root *g(a)lag- "milk") + -al (1). Other 17c. attempts at an adjective in English yielded lactary, lactaceous, lacteant, lacteous, lactescent, and, in a specialized sense ("milk-producing"), lactific.
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