Etymology
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lettuce (n.)
garden herb extensively cultivated for use as a salad, late 13c., letuse, probably somehow from Old French laitues, plural of laitue "lettuce" (cognate with Spanish lechuga, Italian lattuga), from Latin lactuca "lettuce," from lac (genitive lactis) "milk" (from PIE root *g(a)lag- "milk"); so called for the milky juice of the plant. Old English had borrowed the Latin word as lactuce.
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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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romaine (adj.)

type of lettuce, 1876, from French romaine (in laitue romaine, literally "Roman lettuce"), from fem. of Old French romain "Roman," from Latin Romanus (see Roman). Ayto ("Diner's Dictionary," 1990) defines it as "The American term for the long-leaved lettuce usually known to British-speakers as the cos lettuce," and writes that the name probably arose because it was imported into Western Europe from the eastern Mediterranean at first via the port of Rome. He compares Italian lattuga romana and notes that "the earliest English term for it, dating from the beginning of the seventeenth century, was Roman lettuce."

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BLT (n.)

also B.L.T., type of sandwich, initialism for bacon, lettuce, and tomato, the ingredients; 1940s, American English.

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escarole (n.)

"lettuce-like salad vegetable" (a type of endive), 1897, from French escarole (13c., scariole), from Italian scariola, from Medieval Latin escariola "something edible," ultimately from Latin edere "to eat" (from PIE root *ed- "to eat").

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core (n.)

early 14c., "heart or inmost part of anything" (especially an apple, pear, etc.), of uncertain origin, probably from Old French cor, coeur "core of fruit, heart of lettuce," literally "heart," from Latin cor "heart," from PIE root *kerd- "heart."

Meaning "a central portion cut and removed" (as from a tree, soil, etc.) is from 1640s. Meaning "internal mold of a casting, which fills the space intended to be left hollow" is from 1730. Nuclear physics sense "portion of a reactor containing the nuclear fuel and where the reactions take place" is from 1949.

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iceberg (n.)

1774, "glacier humped like a hill;" 1820 as "detached piece of a glacier or ice pack at sea," partial loan-translation of Dutch ijsberg, literally "ice mountain," from ijs "ice" (see ice (n.)) + berg "mountain" (from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts.). Similar formation in Norwegian isberg, Danish isbjerg.

Earlier English terms were sea-hill (1690s), island of ice (1610s). Phrase tip of the iceberg in a figurative sense (in allusion to most of it being unseen underwater) first recorded 1962. Iceberg lettuce attested from 1893, apparently originally a trade name.

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endive (n.)

in Britain, "lettuce-like salad plant of the daisy family;" in U.S., "blanched shoots of Cichorium intybus" (a plant related to the Cichorium endiva, the British "endive"), late 14c., from Old French endive (14c.), from Medieval Latin endivia or a related Romanic source, from Latin intibus. This probably is connected in some way with Medieval Greek entybon, which Klein says is perhaps of Eastern origin (compare Egyptian tybi "January," the time the plant grows in Egypt). Century Dictionary says Arabic hindiba is "appar. of European origin."

Few culinary terms cause such confusion as endive and chicory. The basic problem is that what the British call endive the Americans call chicory, and what the British call chicory the Americans call endive (the French side with the Americans: British endive translates as French chicorée). [Ayto, "Diner's Dictionary"]

The original sense of the word in Middle English was the same as the modern American one, but when the Cichorium endiva, distinguished by its annual root, much longer unequal pappus, and less bitter taste, arrived in Europe from Asia in the 16c., the confusion of names began.

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salad (n.)

late 14c., salade, "raw herbs cut up and variously dressed," from Old French salade (14c.) and Medieval Latin salata, both from Vulgar Latin *salata, literally "salted," short for herba salata "salted vegetables" (vegetables seasoned with brine, a popular Roman dish), from fem. past participle of *salare "to salt," from Latin sal (genitive salis) "salt" (from PIE root *sal- "salt").

Dutch salade, German Salat, Swedish salat, Russian salat are from Romanic languages. Later extended to dishes composed of meat chopped and mixed with uncooked herbs and variously seasoned (chicken salad, etc.). In reference to the raw herbs and vegetables themselves, in U.S. it is colloquially limited to lettuce (by 1838).

Salad oil "olive oil used for dressing salads," is by 1550s. Salad-fork is by 1808. Salad bar is attested by 1940, American English. Salad days "time of youthful inexperience" (perhaps on notion of "green") was used by Shakespeare ("Antony and Cleopatra," 1606) and owes its survival, if not its existence, to him.

Whether the point is that youth, like salad, is raw, or that salad is highly flavoured & youth loves high flavours, or that innocent herbs are youth's food as milk is babes' & meat is men's, few of those who use the phrase could perhaps tell us ; if so, it is fitter for parrots' than for human speech. [Fowler]
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