reredos n.

"hindmost part, the space behind or at the back," c. 1600, abstracted from rerewarde "rear guard, hindmost part of an army or fleet" (mid-14c.), from Anglo-French rerewarde, Old French rieregarde, from the Old French adverb riere "behind" (from Latin retro "back, behind;" see retro-) + Old French garde "guardian" (see guard (n.)).

Earliest use often is specifically military, "hindmost body of an army or fleet." The English word in many early examples also may be a shortened form of arrear (see arrears), perhaps a misdivision of the arrears.

As a euphemism for "buttocks" it is attested by 1796. As an adverb, "behind," early 15c. As an adjective, "hindmost; pertaining to or situated in the rear," c. 1300, from Old French rere.

To bring up the rear "come last in order" is from 1640s. The naval rank of rear admiral is attested from 1580s, said to be so called from his originally ranking "behind" an admiral proper. Rear-view (mirror) is recorded from 1926. Rear-supper (c. 1300) was an old name for "last meal of the day."

"bundle of documents referring to some matter," 1880 (by 1868 as a French word in English), from French dossier "bundle of papers," from dos "back" (12c.), from Vulgar Latin *dossum, variant of Latin dorsum "back" (see dorsal). Supposedly so called because the bundle bore a label on the back, or possibly from resemblance of the bulge in a mass of bundled papers to the curve of a back. Old French dossiere meant "back-strap, ridge strap (of a horse's harness)."