"hindmost part," c.1600, abstracted from
rerewarde "rear guard" (c.1300), from Anglo-Fr.
rerewarde, O.Fr.
rieregarde, from O.Fr.
riere (from L.
retro "back, behind") + O.Fr.
garde (see
guard). Or the word may be an aphetic form of
arrear (see
arrears). Military sense of "hindmost part" of an army or fleet is recorded from 1606. As a euphemism for "buttocks" it is attested from 1796 (
rear end in this sense recorded from 1937).
Rear admiral is first attested 1587, apparently so called from ranking "behind" an admiral proper.
Rear-view (mirror) is recorded from 1926.