Etymology
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lag (v.)
"move slowly, fail to keep pace," 1520s, earlier as a noun meaning "last person" (1510s), later also as an adjective, "slow, tardy, coming behind" (1550s, as in lag-mon "last man"). All are of uncertain relationship and origin, possibly from a Scandinavian source (compare Norwegian lagga "go slowly"), or some dialectal version of last, lack, or delay. Related: Lag; lagging.
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lag (n.)

in the mechanical sense "retardation of movement," 1855, from lag (v.). Also noted in Farmer and Henley ("Slang and Its Analogues") as American theatrical slang for "a wait," with an attestation from 1847. First record of lag time is from 1951.

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jet lag (n.)
also jetlag, 1966, from jet (n.1) in the "airplane" sense + lag (n.). Also known in early days as time zone syndrome.
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laggard (adj.)
1702, "slow, sluggish," from lag (v.) + -ard. From 1757 as a noun, "one who lags, a shirker, loiterer." Related: Laggardly.
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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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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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ablactation (n.)
Origin and meaning of ablactation

"weaning of a child," 1650s, from Latin ablactationem (nominative ablactatio) "weaning," noun of action from past-participle stem of ablactare "to wean," from ab "off, away from" (see ab-) + lactare "to suckle," from lac (genitive lactis) "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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latte (n.)
espresso coffee with milk, by 1990, short for caffè latte, which is an Italian expression meaning literally "milk coffee," from Latin lac (genitive lactis) "milk" (from PIE root *g(a)lag- "milk"). Compare cafe au lait.
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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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