A back-stabber is somebody who attacks you "from behind" where you can't "see" him. That means, he might give you the impression that he is your friend, while in reality it's just the opposite - he's your enemy!
Replying to [Cooleyes]: Westerners know the word Backstabber as someone who is Gay, someone who trafficks Narcotics with anal delivery, or somone who frequents Outback Steakhouses of Australian origin.
Replying to [Cooleyes]:It means being friendly to someone, to their face. And then speaking bad about them when they are not around. Another way to look at it is to think of someone smiling and patting you on the back. Only they are smiling at you while they stab you. I hope this helps!