From Wiktionary, the free dictionary
Borrowed from Middle English bak-biten, bacbiten, equivalent to back- + bite. Cognate with Icelandic bakbíta (“to backbite, vilify”).
bacbîtya
- to backbite
c. 1560, Homelyes XIII in Cornysche:cowsow da an rena neb ara agys defamia hagys bacbytya ha cowse drog a hanowgh why- speak well of those who will defame you and backbite you and speak ill of you
