Etymology
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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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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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pack (v.)

late 14c., pakken, "to put together in a pack, bundle (something) up," from pack (n.), possibly influenced by Anglo-French empaker (late 13c.) and Medieval Latin paccare "pack," both of which are from Germanic (compare Middle Dutch packen).

Meaning "pack compactly, cram or crowd together" is from mid-15c. Sense of "to fill (a container) with things arranged more or less methodically" is from late 15c. Meaning "to go away, leave" is from mid-15c. Meaning "to force or press down or together firmly" (of dirt, snow, etc.) is by 1850.

Some senses suggesting "make secret arrangement, manipulate so as to serve one's purposes" are from an Elizabethan mispronunciation of pact, as in pack the cards (1590s) "arrange the deck so as to give one undue advantage." The sense of "to carry or convey in a pack" (1805) led to the general sense of "to carry in any manner;" hence "to be capable of delivering" (a punch, etc.), attested from 1921, and  pack heat "carry a gun," 1940s underworld slang. To pack it up "give up, finish" is by 1942. Related: Packed; packing.

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pack (n.)

early 13c., pak, pake, "a bundle or package (of cloth, merchandise, etc.)," also "a bag or purse for carrying things," probably from a Low German word (compare Middle Dutch pac, pack "bundle," Middle Low German pak, Middle Flemish pac, attested from late 12c.) and taken into English from the wool traders in Flanders; or possibly from Old Norse pakki. All are of unknown origin. Italian pacco is a Dutch loan word; French pacque probably is from Flemish.

Especially a bundle enclosed in a wrapping and bound fast with cords. Meaning "set of persons" (usually of a low character) is from late 14c. and is older than sense of "group of instinctively herding hunting animals" (mid-15c.). Extended to "complete set of playing cards" (1590s), floating ice (1791), bundled cigarettes (1865), and submarines (1943).

Meaning "knapsack on a frame" is attested from 1916. Pack of lies is attested from 1763. Meaning "a person of low character" (usually with naughty) is by 1520s.

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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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pack-horse (n.)

"horse used in carrying burdens," c. 1500, from pack (n.) + horse (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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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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