"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).
end n.
Old English ende "end, conclusion, boundary, district, species, class," from Proto-Germanic *andiaz (source also of Old Frisian enda, Old Dutch ende, Dutch einde, Old Norse endir "end;" Old High German enti "top, forehead, end," German Ende, Gothic andeis "end"), originally "the opposite side," from PIE *antjo "end, boundary," from root *ant- "front, forehead," with derivatives meaning "in front of, before."
Worldly wealth he cared not for, desiring onely to make both ends meet. [Thomas Fuller, "The History of the Worthies of England," 1662]
Original sense of "outermost part" is obsolete except in phrase ends of the earth. Sense of "destruction, death" was in Old English. Meaning "division or quarter of a town" was in Old English. The end "the last straw, the limit" (in a disparaging sense) is from 1929. The end-man in minstrel troupes was one of the two at the ends of the semicircle of performers, who told funny stories and cracked jokes with the middle-man. U.S. football end zone is from 1909 (end for "side of the field occupied by one team" is from 1851). The noun phrase end-run is attested from 1893 in U.S. football; extended to military tactics by 1940. End time in reference to the end of the world is from 1917. To end it all "commit suicide" is attested by 1911. Be-all and end-all is from Shakespeare ("Macbeth" I.vii.5).