"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."
Middle English reren, from Old English ræran "to raise, lift something, cause to rise;" also "to build up, create, set on end; to arouse, excite, stir up," from Proto-Germanic *raizijanau "to raise," causative of *risanan "to rise" (source of Old English risan; see rise (v.)). The second -r- is by rhotacism.
Meaning "bring into being, bring up" (as a child) is recorded by early 15c., perhaps late 14c.; at first it is not easy to distinguish the sense from simply "beget;" the meaning "bring up (animals or persons) by proper nourishment and attention, develop or train physically or mentally" had developed by late 16c.
The intransitive meaning "raise up on the hind legs" is first recorded late 14c. (compare rare (v.)). As what one does in raising or holding high the head, by 1667 ("Rear'd high thir flourisht heads" - Milton); with ugly by 1851. Related: Reared; rearing.
Other uses of rear in Middle English were "set" (fire); "draw" (blood); "wage" (war); "raise" (revenue, tithes); "gather, collect" (a flock of sheep).