Etymology
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leak (n.)
"hole by which liquid enters or escapes," late 15c., from leak (v.) or Old Norse cognate leka. Sense of "revelation of secret information" is from 1950. Meaning "act of urination" is attested from 1934 ("Tropic of Cancer"); but the verb meaning "to piss" is from 1590s: "Why, you will allow vs ne're a Iourden [i.e. a chamberpot], and then we leake in your Chimney." ["I Hen. IV," II.i.22]
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leak (v.)
"to let water in or out" [Johnson], late 14c., from Middle Dutch leken "to drip, to leak," or from Old Norse leka, both of them related to Old English leccan "to moisten, water, irrigate" (which did not survive into Middle English), all from Proto-Germanic *lek- "deficiency" (source also of Old High German lecchen "to become dry," German lechzen "to be parched with thirst"), from PIE root *leg- (2) "to dribble, trickle." The figurative meaning "come to be known in spite of efforts at concealment" dates from at least 1832; transitive sense first recorded 1859. Related: Leaked; leaking.
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leaky (adj.)
mid-15c., from leak (n.) + -y (2). Related: Leakiness. Slang sense of "unable to keep a secret" attested from 1704.
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lack (n.)
c. 1300, "absence, want; shortage, deficiency," not found in Old English, of uncertain origin. Perhaps it is from an unrecorded Old English *lac, or else borrowed from Middle Dutch lak "deficiency, fault;" in either case probably from Proto-Germanic *lek- (source also of Old Frisian lek "disadvantage, damage," Old Norse lakr "lacking" (in quality), "deficient" (in weight)), from PIE *leg- (2) "to dribble, trickle" (see leak (v.)). Middle English also had lackless "without blame or fault."
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leach (v.)

"wash or drain by percolation of water, treat by downward drainage," by 1660s in cookery, perhaps from a dialectal survival from Old English leccan "to moisten, water, wet, irrigate," which, under Norse influence, became leak (v.). The word was used 18c. in technological senses, such as leach-trough, a device used in salt-works in which corns of salt taken from brine were set to drain  dry, after which they were called leach-brine. Related: Leached; leaching. Hence leach (n.) "a preparation made by leaching or straining" (1630s), in later use especially "a separation of lye or alkali in solution."

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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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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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transpire (v.)

1590s, "pass off in the form of a vapor or liquid," from French transpirer (16c.), from Latin trans "across, beyond; through" (see trans-) + spirare "to breathe" (see spirit (n.)). Figurative sense of "leak out, become known" is recorded from 1741, and the erroneous meaning "take place, happen" is almost as old, being first recorded 1755. Related: Transpired; transpiring.

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stopgap (n.)

also stop-gap, 1680s, from stop (v.) + gap (n.); the notion probably being of something that plugs a leak, but it may be in part from gap (n.) in a specific military sense "opening or breach in defenses by which attack may be made" (1540s). Also as an adjective from 1680s.

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