Etymology
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dusky (adj.)

1550s, "somewhat dark, not luminous, dim;" see dusk + -y (2). "The normal source of an adj. in -y is a sb.; but the substantival use of dusk is not known so early as the appearance of dusky, so that the latter would appear to be one of the rare instances of a secondary adj. ..." [OED]. Meaning "rather black, dark-colored" is from 1570s. Related: Duskily; duskiness.

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wood (n.)
Old English wudu, earlier widu "tree, trees collectively, forest, grove; the substance of which trees are made," from Proto-Germanic *widu- (source also of Old Norse viðr, Danish and Swedish ved "tree, wood," Old High German witu "wood"), from PIE *widhu- "tree, wood" (source also of Welsh gwydd "trees," Gaelic fiodh- "wood, timber," Old Irish fid "tree, wood"). Out of the woods "safe" is from 1792.
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wood (adj.)
"violently insane" (now obsolete), from Old English wod "mad, frenzied," from Proto-Germanic *woda- (source also of Gothic woþs "possessed, mad," Old High German wuot "mad, madness," German wut "rage, fury"), from PIE *wet- (1) "to blow; inspire, spiritually arouse;" source of Latin vates "seer, poet," Old Irish faith "poet;" "with a common element of mental excitement" [Buck]. Compare Old English woþ "sound, melody, song," Old Norse oðr "poetry," and the god-name Odin.
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rat (v.)

1812, "to desert one's party, go over from a losing cause;" 1847 as "to work for less than current wages, refuse to join a labor strike;" 1864 as "to catch or kill rats;" 1910 as "to peach on, inform on, behave dishonestly toward;" from rat (n.). All but the third are extended from the proverbial belief that rats leave a ship about to sink or a house about to fall. Related: Ratted; ratting.

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rat (n.)

"a rodent of some of the larger species of the genus Mus," late Old English ræt "rat," a word of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Medieval Latin ratus, Italian ratto, Spanish rata, Old French rat) and Germanic (Old Saxon ratta; Middle Dutch ratte, Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but their connection to one another and the ultimate source of the word are unknown. In its range and uncertain origin, it is much like cat.

Perhaps from Vulgar Latin *rattus, but Weekley thinks this is of Germanic origin, "the animal having come from the East with the race-migrations" and the word passing thence to the Romanic languages. American Heritage and Tucker connect Old English ræt to Latin rodere and thus to PIE root *red- "to scrape, scratch, gnaw," source of rodent (q.v.). Klein says there is no such connection and suggests a possible cognate in Greek rhine "file, rasp." Weekley connects the English noun and the Latin verb with a question mark and OED says it is "probable" that the rat word spread from Germanic to Romanic, but takes no position on further etymology. The common Middle English form was ratton, from augmented Old French form raton. Applied to rat-like species on other continents from 1580s.

The distinction between rat and mouse, in the application of the names to animals everywhere parasitic with man, is obvious and familiar. But these are simply larger and smaller species of the same genus, very closely related zoologically, and in the application of the two names to the many other species of the same genus all distinction between them is lost. [Century Dictionary]

Applied since 12c. (in surnames) to persons held to resemble rats or share some characteristic or quality with them. Specific sense of "one who abandons his associates for personal advantage" (1620s) is from the belief that rats leave a ship about to sink or a house about to fall, and this led to the meaning "traitor, informant" (1902).

To smell a rat "to be put on the watch by suspicion as the cat by the scent of a rat; to suspect danger" [Johnson] is from 1540s.  _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.

RATS. Of these there are the following kinds: a black rat and a grey rat, a py-rat and a cu-rat. ["Dictionary of the Vulgar Tongue," Grose, 1788]  
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cord-wood (n.)

"cut wood sold by the cord for fuel," 1630s, from cord in the wood-measure sense + wood (n.).

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four-footed (adj.)
c. 1300, fourefoted; see four + foot (n.). Replacing forms from Old English feowerfote.
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heart-wood (n.)
also heartwood, 1801, from heart (n.) in the sense "central part of a tree" (c. 1400) + wood (n.).
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rat-terrier (n.)

"small, active dog used to kill rats," by 1852, American English, from rat (n.) + terrier.

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flat-footed (adj.)
c. 1600, "with flat feet;" see flat (adj.) + foot (n.). Meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial adverbial use it meant "straightforwardly, downright, resolute" (1828), from notion of "standing firmly."
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