Etymology
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rat-tail (n.)

also rat's-tail, from rat (n.) + tail (n.1). Used since 16c. of conditions, growths, or devices held to resemble a rat's long, hairless tail in any way, including "lank lock of hair" (1810); "end of a rope" (1867). Related: Rat-tailed. A rat-tail file (1744) is a fine, round file used for enlarging holes in metal.

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rugrat (n.)

also rug-rat, "baby, small child," by 1968; see rug + rat (n.).

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rats (interj.)

expressing incredulity, disappointment, annoyance, etc., 1886, American English, from rat (n.).

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rat-race (n.)

also rat race, 1934 in reference to aviation training, from rat (n.) + race (n.1).

A rat race is ... a simple game of "follow the leader" in fighter planes. The leader does everything he can think of — Immelmanns, loops, snap rolls, and turns, always turns, tighter and tighter. [Popular Science, May 1941]

In the 1930s actual rat races of some sort are frequently mentioned among popular carnival and gambling attractions. Meaning "fiercely competitive struggle," especially to maintain one's position in work or life is by 1939. Rat-run is from 1870 in the sense of "maze-like passages by which rats move about their territory," but originally and usually in a derogatory transferred sense.

[Matthew] Milton was not, at the period we write of [c. 1811], at all in the ring ; for in the following March he was steward of a rat-race, held at a public-house in Shepherd's-market, where four of these "varmin," decorated with different coloured ribands, were started for a sweepstakes, round the clubroom, before a host of sportsmen. ["Sporting Incidents at Home and Abroad," The Sporting Review, May 1848]
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ratbag (n.)
also rat-bag, "unpleasant person," 1937, from rat (n.) + bag (n.).
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ratty (adj.)

1856, "resembling a rat;" 1865, "full of rats;" 1867, "wretched, miserable, shabby," from rat (n.) + -y (2). An older word for "resembling a rat" is rattish (1680s).

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ratsbane (n.)

"rat poison, arsenic," 1520s; see rat (n.) + bane. Compare henbane, fleabane, wolfsbane.

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rodent (n.)

"a rodent mammal" 1835 (as an adjective 1833), from Modern Latin Rodentia, the order name, from Latin rodentem (nominative rodens), "the gnawers," present participle of rodere "to gnaw, eat away," which is of uncertain etymology, possibly is from an extended form of PIE root *red- "to scrape, scratch, gnaw." Uncertain connection to Old English rætt (see rat (n.)). They are characterized by having no canine teeth and strong incisors.

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rat-a-tat (n.)

"a rattling sound or effect," 1680s, echoic, originally of a cooper hammering tubs.

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pack-rat (n.)
common name for the North American bushytailed woodrat (Neotoma cinerea) 1885, from pack (v.); so called from the rodents' habit of dragging objects off to their holes. Used figuratively or allusively from c. 1850 of persons who won't discard anything, which means either the rat's name is older than the record or the human sense is the original one.
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