"Stabbing someone in the back" means betraying them. In other words, you act friendly toward them but secretly do things to hurt them.

Some examples of "stabbing someone in the back" include:

• sleeping with your close friend's girlfriend or boyfriend
• revealing an embarassing secret that someone asked you not to tell
• taking a job that you know that another person wanted and that they told you about

 The phrase "stab ___ in the back" is very emotional and dramatic. People actually don't use it very much to directly confront someone. It's more common to use "stab ___ in the back" when telling stories about the mean things that people have done to you.

We call a person who does this "a backstabber".