from The American Heritage® Dictionary of the English Language, 4th Edition
- transitive v. To attack (someone) unfairly, especially in an underhand, deceitful manner: "Some backstab each other and threaten to settle their differences with a punch” ( Thomas Boswell).
from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. to attack someone (especially verbally) unfairly in a deceitful, underhand, or treacherous manner, especially when they're not present in the place or situation that it happens. (as if stabbing them in the back). See backbite.
back + stab (Wiktionary)