Stab in the back

My colleague has just asked if there is a meaning to a phrase we have both heard and used. It is:

Stab you in the back as soon as look at you

To "stab you in the back" means the person will betray a confidence for personal gain, without regard to the consequences you will suffer. "As soon as look at you" means they will do it without hesitation.

I understand "stab ... in the back" to have a broader meaning than betraying confidences. The person will do something sneaky that hurts you: starting false rumors, taking credit for your work, seducing your spouse, anything. ~rb