Etymology
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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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rat-a-tat (n.)

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

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rat-catcher (n.)

"one whose business is the catching of rats, a ratter," 1590s, from rat (n.) + catcher.

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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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rat-snake (n.)

"snake which kills rats," 1818, from rat (n.) + snake (n.).

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rat-hole (n.)

also rathole, "the hole gnawed in woodwork, etc., by a rat or rats," 1812 first in the figurative sense of "nasty, messy place;" from rat (n.) + hole (n.). As "bottomless hole" (especially one where money goes) by 1921.

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rat-pack (n.)

1951, "a gang of disorderly young people" [OED], from rat (n.) + pack (n.). In reference to the Hollywood circle around Frank Sinatra, from 1958.

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rat-trap (n.)

"trap for catching rats," late 15c., rat trappe, from rat (n.) + trap (n.).

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rat-poison (n.)

"something used to poison rats with," especially arsenic, 1799, from rat (n.) + poison (n.).

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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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