"hindmost part of an animal," Old English tægl, tægel, from Proto-Germanic *tagla- (cf. Old High German zagal, German Zagel "tail," dialectal German Zagel "penis," Old Norse tagl "horse's tail"), from PIE *doklos, from root *dek- "something long and thin" (referring to such things as fringe, lock of hair, horsetail; cf. Old Irish dual "lock of hair," Sanskrit dasah "fringe, wick"). 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. Another Old English word for "tail" was steort (see stark).
Meaning "reverse side of a coin" 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 "prostitute" (1846). 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.