Something someone does to you that's messed up. Usually a promise or agreement that is broken when the person who broke the promise or agreement knew he was going to break it and did not tell you until it was too late.
Josh: "Hey Brian. Can you drive me to the train station at 4:30?"
Brian:"Yea man no problem"
Josh: (Later)"Hey Brian it's 4:45 where are you?"
Brian: "O yea my bad man I went over my new girlfriends house."
Josh: Don't you think you should of told me that, I'm gonna miss my train now.
Brian: My bad
Josh: Wow man that's pretty shitty.