Etymology
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fin (n.)

Old English finn "fin," from Proto-Germanic *finno (source also of Middle Low German vinne, Dutch vin), perhaps from Latin pinna "feather, wing" (see pin (n.)); or, less likely, from Latin spina "thorn, spine" (see spine).

U.S. underworld slang sense of "$5 bill" is 1925, from Yiddish finif "five," from German fünf (from PIE root *penkwe- "five") and thus unrelated. The same word had been used in England in 1868 to mean "five pound note" (earlier finnip, 1839).

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tail (n.1)

"hindmost part of an animal," Old English tægl, tægel "a tail," from Proto-Germanic *tagla- (source also of Old High German zagal, German Zagel "tail," dialectal German Zagel "penis," Old Norse tagl "horse's tail," Gothic tagl "hair"), from PIE *doklos, from suffixed form of root *dek- (2) "something long and thin" (referring to such things as fringe, lock of hair, horsetail; source also of Old Irish dual "lock of hair," Sanskrit dasah "fringe, wick").

According to OED, the primary sense, at least in Germanic, seems to have been "hairy tail," or just "tuft of hair," but already in Old English the word was applied to the hairless "tails" of worms, bees, etc. But Buck writes that the common notion is of "long, slender shape." As an adjective from 1670s.

Meaning "reverse side of a coin" (opposite the side with the head) is from 1680s; that of "backside of a person, buttocks" is recorded from c. 1300; slang sense of "pudenda" is from mid-14c.; that of "woman as sex object" is from 1933, earlier "act of copulation" with a prostitute (1846). Of descending strokes of letters, from 1590s.

Tails "coat with tails" is from 1857. The tail-race (1776) is the part of a mill race below the wheel. To turn tail "take flight" (1580s) originally was a term in falconry. The image of the tail wagging the dog is attested from 1907. Another Old English word for "tail" was steort (see stark).

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tail (n.2)

"limitation of ownership," a legal term, early 14c. in Anglo-French; late 13c. in Anglo-Latin, in most cases a shortened form of entail.

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tail (v.)

1520s, "attach to the tail," from tail (n.1). Meaning "move or extend in a way suggestive of a tail" is from 1781. Meaning "follow secretly" is U.S. colloquial, 1907, from earlier sense of "follow or drive cattle." Related: Tailed; tailing. Tail off "diminish" is attested from 1854.

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tail-hook (n.)

1861, in angling, from tail (n.1) + hook (n.).

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tail-gate (n.)

1868, back panel on a wagon, hinged to swing down and open, from tail (n.1) + gate (n.). Extended by 1950 to hatchback door on an automobile. The verb (also tailgate) meaning "to drive too close behind another vehicle" is from 1951 ("Truck drivers know the practice of following too close as tail-gating" - "Popular Science," Jan. 1952); as an adjective, in reference to the open tail-gate of a parked car as a setting for a party or picnic, from 1958. Related: Tail-gating.

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white-tail (n.)

type of North American deer, 1872, from white (adj.) + tail (n.).

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tail-end (n.)

late 14c., from tail (n.1) + end (n.).

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tail-pipe (n.)

also tailpipe, 1757, "small pipe fixed at the swell of a musket to receive the ramrod," from tail (n.1) + pipe (n.). From 1832 as "suction pipe of a pump;" 1907 as "exhaust pipe of an automobile."

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coat-tail (n.)

c. 1600, "flaps formed by the lower back of a coat," from coat (n.) + tail (n.). In 17c., to do something on one's own coattail meant "at one's own expense." Meaning "power of one person," especially in politics, is at least from 1848 (in a Congressional speech by Abraham Lincoln); expression riding (someone's) coattails into political office is from 1949.

But the gentleman from Georgia further says we [Whigs] have deserted all our principles, and taken shelter under General Taylor's military coat-tail, and he seems to think this is exceedingly degrading. Well, as his faith is, so be it unto him. But can he remember no other military coat-tail under which a certain other party have been sheltering for near a quarter of a century? Has he no acquaintance with the ample military coat-tail of General Jackson? Does he not know that his own party have run the five last presidential races under that coat-tail? And that they are now running the sixth under the same cover? Yes, sir, that coat-tail was used not only for General Jackson himself, but has been clung to, with the grip of death, by every Democratic candidate since. [Lincoln, speech in Congress, July 27, 1848]
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