Английский Этимологический словарь - rear (n.)

- early 14c., abstracted from rerewarde "rear guard," from Anglo-Fr. rerewarde, O.Fr. rieregarde, from L. retro "back, behind" + O.Fr. garde (see guard). Or the word may be an aphetic form of arrear. Rear admiral is first attested 1587. Rear end as euphemism for "buttocks" is from 1937.