Etymology
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*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."

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