Etymology
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lake (n.1)
"body of water surrounded by land and filling a depression or basin," early 12c., from Old French lack (12c., Modern French lac) and directly from Latin lacus "pond, pool, lake," also "basin, tank, reservoir" (related to lacuna "hole, pit"), from PIE *laku- "body of water, lake, sea" (source also of Greek lakkos "pit, tank, pond," Old Church Slavonic loky "pool, puddle, cistern," Old Irish loch "lake, pond"). The common notion is "basin."

There was a Germanic form of the PIE root which yielded Old Norse lögr "sea flood, water," Old English lacu "stream, pool, pond," lagu "sea flood, water, extent of the sea," leccan "to moisten" (see leak (v.)). In Middle English, lake, as a descendant of the Old English word, also could mean "stream; river gully; ditch; marsh; grave; pit of hell," and this might have influenced the form of the borrowed word.
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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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lake (v.)
"to play, sport," Old English lacan (see lark (n.2)).
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lacustrine (adj.)
"of or pertaining to lakes," 1826, irregularly formed from Latin lacus "lake" (see lake (n.1)).
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lough (n.)
"a lake, pool," early 14c., Anglo-Celtic, representing a northern form of Irish and Gaelic loch, Welsh llwch, from PIE *laku- (see lake (n.1), and compare loch).
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laker (n.)
a word used of people or things associated in various ways with a lake or lakes, including tourists to the English Lake country (1798); the poets (Wordsworth, etc.) who settled in that region (1814); boats on the North American Great Lakes (1887), and a person whose work is on lakes (1838); see lake (n.1). The U.S. professional basketball team began 1947 as the Minneapolis Lakers, where the name was appropriate; before the 1960-1 season it moved to Los Angeles, but kept the name.
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lacuna (n.)
"blank or missing portion in a manuscript," 1660s, from Latin lacuna "hole, pit," figuratively "a gap, void, want," diminutive of lacus "pond, lake; hollow, opening" (see lake (n.1)). The Latin plural is lacunae. The word has also been used in English from c. 1700 in the literal Latin sense in anatomy, zoology, botany. The adjectival forms have somewhat sorted themselves: Mathematics tends to use lacunary (1857), natural history lacunose (1816), and lacunar (n.) is used in architecture of paneled ceilings (1690s), so called for their sunken compartments. Leaving lacunal (1846) for the manuscript sense.
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loch (n.)

late 14c., from Gaelic loch "lake, lake-like body," including the narrow, nearly land-locked arms of the sea found in the glacier-scoured landscape of west Scotland; cognate with Old Irish loch "body of water, lake," Breton lagen, Anglo-Irish lough, Latin lacus (see lake (n.1)). "The word was adopted in ONorthumbrian as luh" [OED]. The diminutive form is lochan.

The phrase Loch Ness monster is attested by 1934, the thing itself under slightly different names from 1933. The loch is named for the river Ness that flows out of it at Inverness; the river name is probably from an Old Celtic word meaning "roaring one."

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lagoon (n.)

1670s, lagune, earlier laguna (1610s), "area of marsh or shallow, brackish water beside a sea but separated from it by dunes," from French lagune or directly from Italian laguna "pond, lake," from Latin lacuna "pond, hole," from lacus "pond" (see lake (n.1)). Originally in reference to the region of Venice. The word was applied 1769 (by Capt. Cook) to the lake-like stretch of water enclosed in a South Seas atoll. Also see -oon. Related: Lagoonal.

In regions where Spanish is or formerly was the current language, the word lagoon is likely to be used with more latitude of meaning, since in the Spanish laguna is applied to ordinary lakes, to the bottoms of deep bays, especially when these are more or less closed in by a narrowing of the coast-lines, so as to give rise to lake-like areas, and also to shallow, swampy, or almost dried-up lakes inland as well as near the coast. [Century Dictionary]
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scarlet (n.)

mid-13c. (late 12c. as a surname), "rich cloth" (often, but not necessarily, bright red), from a shortened form of Old French escarlate "scarlet (color), top-quality fabric" (12c., Modern French écarlate), which, with Medieval Latin scarlatum "scarlet, cloth of scarlet," Italian scarlatto, Spanish escarlate often is said to be from a Middle Eastern source, but perhaps is rather from a Germanic source akin to Old High German scarlachen, scharlachen (c. 1200), from scar "sheared" + lachen "cloth."

In English it is attested as the name of a color, a highly chromatic and brilliant red, from late 14c. It was used as an adjective in reference to this color, or to gowns of this color, from c. 1300.

Scarlet Lady is Biblical (Isaiah i.18, Revelation xvii.1-5); she has been variously identified by commentators. Scarlet woman "notoriously immoral woman, prostitute" (by 1924) perhaps is from notion of "red with shame or indignation." Earlier it was used in the same sense as Scarlet Lady.

Scarlet fever is from 1670s, so called for its characteristic rash. It also was an old slang term for the condition of women irresistibly glamoured by men in uniform. Scarlet oak, a New World tree, is so called from 1590s. Scarlet letter in figurative use traces to Hawthorne's story (1850), a red cloth "A" which convicted adulterers were condemned to wear. German Scharlach, Dutch scharlaken show influence of words cognate with English lake (n.2).

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