Where is it from and how did the phrase originate? Etymonline only lists that it dates back to the thirteenth century and that The notion is of injury in a manner comparable to biting from behind. and that old English had bæcslitol as an adjective meaning a similar thing. What I am wondering is whether it has connection to or is from Arabic, as the act is compared to "eating of the flesh of your brother" in Arabic as well, dating back to the seventh century. Or is it possible that it is just a common metaphor that developed independently in different languages and cultures?
The Germanic Lexicon Project has Bosworth-Toller available:
bæc-slitol, es; m. [bæc a back; slitol a biter, from sliten, pp. of slítan to slit, bite] A backbiter; detractor, Off. reg. 15.
slítan, IV :-- Ða slítnysse gedígean a laceratione (by wolves or dogs) convalescere, L. Ecg. C. 40; Th. ii. 166, 25. II. a wasting, destroying, desolation, v. slítan, VI :-- Slítnese desolationis, Mt. Kmbl. Lind. 24, 15. v. from-, tó-slítness.
slitol; adj. I. pungent, biting, v. slítan. V :-- Slitul léc mordax allium, Germ. 394, 260.
II. carping, backbiting, v. bæc-slitol, slítan, VII.
slítung, e; f.
I. tearing, rending, biting, v. slítan, IV :-- Slítinc &l-bar; geter dilaceratio. Hpt. Gl. 499, 21. Fugelas hig fretaþ mid ðære biterustan slítunge devorabunt eos aves morsu amarissimo, Dent. 32, 24,
Sume men fram ðara wyrma slítunge sweltaþ, Lchdm. ii. 176, 14.
II. wasting, spoiling, v. slítan, VI :-- Slítunge arpagine (or under I ?), Wrt. Voc. ii. 5, 38: 87, 72 (Wright has sutunge). [Prompt. Parv. slytynge consumpcio: O. H. Ger. slízunga saevitia.]
It is not as if OE "bitan" did not mean "to bite":
- Monnan ic ne bíte nymþe he me bíte I bite no man unless he bite me.
Yet "bæc-bitung" is not recorded.
It seems from the examples available that slítan carried the nuance of "painful or "stinging (adj.)", whereas "bitan" was neutral.
Thus, although the concept of people's pungent slandering of one behind or at one's back and causing emotional pain is probably as old as paranoia, and has been expressed in many cultures in many languages, the word "backbite" only entered the English language, as you have observed in Middle English.
Forms: Middle English bac-, bakbite,
Etymology: < back adv. + bite v., i.e. to bite one on, or behind, his back. To detract from the character of, to slander, traduce, speak ill of:
a person absent.
c1175 [implied in: Cott. Hom. 205 Cursunge, bacbitunge and fike~lunge. (at backbiting n.)].
a1300 E.E. Psalter xxxviii. 20 Þat yheldes ivels for godes bac-bate [L. detrahebant] me.
At the risk of being told that this is an "I'll Google that for you" or IGTFY answer and loss of reputation for quoting a source which is less than 100% reliable I will refer you to the Wikipedia entry for "backbiting" which suggests that the term originated as an attack by a dog on a bear being "baited" in a bear pit.
Bear baiting was popular, particularly among the aristocracy and royalty, from the twelfth to the 19th centuries but was eventually banned as inhumane.
Apparently an attack by a dog from behind on a tethered bear was considered cowardly and unsporting although the practice of tethering a bear to a post and setting dogs onto it was considered to be good entertainment so long as the dogs attacked from the front. However this was at a time when executions, including "hanging, drawing and quartering" were held in public and attended by large crowds. The past really is a different country.