Etymology
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Words related to lacto-

*g(a)lag- 
also *g(a)lakt-, Proto-Indo-European root meaning "milk."

It forms all or part of: ablactation; cafe au lait; galactic; galaxy; lactate (v.); lactate (n.); lactation; lacteal; lactescence; lactic; lactivorous; lacto-; lactose; latte; lettuce.

It is the hypothetical source of/evidence for its existence is provided by: Latin lac (genitive lactis) "milk;" Greek gala (genitive galaktos), "milk;" Armenian dialectal kaxc' "milk." The initial "g" probably was lost in Latin by dissimilation. This and the separate root *melg-, account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery.
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*melg- 

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 forms all or part of: emulgent; emulsify; emulsion; milch; milk.

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."

milk (n.)
Origin and meaning of milk

"opaque white fluid secreted by mammary glands of female mammals, suited to the nourishment of their young," Middle English milk, from Old English meoluc (West Saxon), milc (Anglian), from Proto-Germanic *meluk- "milk" (source also of Old Norse mjolk, Old Frisian melok, Old Saxon miluk, Dutch melk, Old High German miluh, German Milch, Gothic miluks), from *melk- "to milk," from PIE root *melg- "to wipe, to rub off," also "to stroke; to milk," in reference to the hand motion involved in milking an animal. Old Church Slavonic noun meleko (Russian moloko, Czech mleko) is considered to be adopted from Germanic.

Of milk-like plant juices or saps from c. 1200. Milk chocolate (eating chocolate made with milk solids, paler and sweeter) is recorded by 1723; milk shake was used from 1889 for a variety of concoctions, but the modern version (composed of milk, flavoring, etc., mixed by shaking) is from the 1930s. Milk tooth (1727) uses the word in its figurative sense "period of infancy," attested from 17c. To cry over spilt milk (representing anything which, once misused, cannot be recovered) is first attested 1836 in writing of Canadian humorist Thomas C. Haliburton. Milk and honey is from the Old Testament phrase describing the richness of the Promised Land (Numbers xvi.13, Old English meolc and hunie). Milk of human kindness is from "Macbeth" (1605).

cafe au lait (n.)
1763, French café au lait, literally "coffee with milk," from lait "milk" (12c.), from Latin lactis, genitive of lac "milk" (see lacto-). As opposed to café noir "black coffee."
lactivorous (adj.)
1824; see lacto- "milk" + -vorous "devouring."