rear (n.) Look up rear at Dictionary.com
"hindmost part," c. 1600, abstracted from rerewarde "rear guard, hindmost part of an army or fleet" (mid-14c.), from Anglo-French rerewarde, Old French rieregarde, from Old French adverb riere "behind" (from Latin retro "back, behind;" see retro-) + Old French garde (see guard (n.)). Or the word may be a shortened form of arrear (see arrears).

As a euphemism for "buttocks" it is attested from 1796. Rear admiral is first attested 1580s, apparently so called from ranking "behind" an admiral proper. Rear-view (mirror) is recorded from 1926.
rear (v.1) Look up rear at Dictionary.com
Old English ræran "to raise, build up, create, set on end; arouse, excite, stir up," from Proto-Germanic *raizijanau "to raise," causative of *risanan "to rise" (see raise (v.)). Meaning "bring into being, bring up" (as a child) is recorded from early 15c.; that of "raise up on the hind legs" is first recorded late 14c. Related: Reared; rearing.
rear (adj.) Look up rear at Dictionary.com
c. 1300, from Old French rere (see rear (n.)).
rear (v.2) Look up rear at Dictionary.com
"attack in the rear," 17c., from rear (n.).