"at or to the rear," 1590s, from rear (adj.) + -ward. It had been used in Middle English as a noun meaning "the part of an army behind the main body" (i.e. "rear guard").
Entries linking to rearward
rear 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."
-ward
adverbial suffix expressing direction, Old English -weard "toward," literally "turned toward," sometimes -weardes, with genitive singular ending of neuter adjectives, from Proto-Germanic *werda- (cognates: Old Saxon, Old Frisian -ward, Old Norse -verðr), variant of PIE *werto- "to turn, wind," from root *wer- (2) "to turn, bend." The original notion is of "turned toward."
*wer- 3
Proto-Indo-European root meaning "perceive, watch out for."
It forms all or part of: Arcturus; avant-garde; award; aware; beware; Edward; ephor; garderobe; guard; hardware; irreverence; lord; panorama; pylorus; rearward; regard; revere; reverence; reverend; reward; software; steward; vanguard; ward; warden; warder; wardrobe; ware (n.) "manufactured goods, goods for sale;" ware (v.) "to take heed of, beware;" warehouse; wary.
It is the hypothetical source of/evidence for its existence is provided by: Latin vereri "to observe with awe, revere, respect, fear;" Greek ouros "a guard, watchman," horan "to see;" Hittite werite- "to see;" Old English weard "a guarding, protection; watchman, sentry, keeper."