Etymology
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lac (n.)

"red resinous substance," 1550s, perhaps immediately from French lacce, displacing or absorbing earlier lacca (early 15c.), from Medieval Latin lacca. All these are from Persian lak, from Hindi lakh (Prakrit lakkha), from Sanskrit laksha "red dye," which is of uncertain origin.

According to Klein, it means literally "one hundred thousand" and is a reference to the insects that gather in great numbers on the trees and create the resin. But others say lakh is perhaps an alteration of Sanskrit rakh, from an IE root word for "color, dye" [Watkins]. Still another guess is that Sanskrit laksha is related to English lax, lox "salmon," and the substance perhaps was so called from being somewhat the color of salmon [Barnhart]. Also see shellac (n.).

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lake (n.2)
"deep red coloring matter," 1610s, from French laque (15c., see lac), from which it was obtained.
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shellac (n.)
also shell lac, 1713, from shell (n.) + lac. Translates French laque en écailles "lac in thin plates." Commercially, lac was considered as stick lac (still on the twigs, insects and all), seed-lac (resin without the twigs and insects, partly processed), and fully processed plates of shell lac.
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lacquer (n.)
1570s, "dye obtained from lac;" 1670s as "gold-colored solution of shellac," from obsolete French lacre, name for a kind of sealing wax, from Portuguese lacre, unexplained variant of lacca "resinous substance," from Arabic lakk, from Persian lak (see lac).
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litmus (n.)
"blue dye-stuff obtained from certain lichens," early 14c., lit-mose, probably from an Old Norse word related to Norwegian dialectal litmose, from Old Norse lita "to dye, to stain" (from litr "color, dye;" see lit (n.1)) + mos "moss." Said to be also in part from Middle Dutch lijkmoes (Dutch lakmoes), from lac (see lac) + moes "pulp." Another idea [Watkins] connects the first element to Middle Dutch leken "to drip, leak" (see leak (v.)). The second element is in any case the common Germanic word for "moss, lichen" (see moss).

The dye is obtained from certain lichens. It is naturally blue but turns red in acid and is restored to blue by alkalis. Figurative use of litmus test is first attested 1957, from scientific use of litmus-treated paper as a chemical indicator. Litmus paper with this meaning is from 1803.
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lacto- 
before vowels, lac-, word-forming element used in chemistry and physiology from 19c. and meaning "milk," from Latin lac (genitive lactis) "milk," from Proto-Italic *(g)lagt-, from PIE root *g(a)lag- "milk." This and the separate root *melg- (source of milk (n.)) account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery. Middle Irish lacht, Welsh llaeth "milk" are loan words from Latin.
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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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lactose (n.)
sugar from milk, 1843, from French, coined 1843 by French chemist Jean Baptiste André Dumas (1800-1884) from Latin lac (genitive lactis) "milk" (from PIE root *g(a)lag- "milk") + chemical suffix -ose (2).
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lactation (n.)

1660s, "process of suckling an infant," from French lactation, from Late Latin lactationem (nominative lactatio) "a suckling," noun of action from past-participle stem of lactare "to suckle," from lac (genitive lactis) "milk" (from PIE root *g(a)lag- "milk"). Meaning "process of secreting milk from the breasts" first recorded 1857. Related: Lactational.

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lactic (adj.)
1790, "procured from milk," in the chemical name lactic acid, which is so called because it was obtained from sour milk. From French lactique, from Latin lactis, genitive of lac "milk" (from PIE root *g(a)lag- "milk.") + French -ique (see -ic).
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