Английский Этимологический словарь - tail (n.)

- O.E. tжgl, tжgel, from P.Gmc. *tagla- (root of dialectal Ger. Zagel "penis"), from PIE *doklos "something long and thin." The verb sense of "follow secretly," recorded in U.S. colloquial use from 1907, is from earlier sense of "follow or drive cattle." Meaning "reverse side of a coin" is from 1684; that of "backside of a person, buttocks" was in M.E. The legal sense (M.E.) is usually from entail. The tail-race (1776) is the part of a mill race below the wheel.