The act of pushing on a skateboard with the wrong foot. If you skate goofy (with your right foot forward,) then you would push mongo if you pushed w/ your right foot. Depending on where you go, mongo pushing is either accepted, or hated. Many beginner level skaters push mongo, so some people see it as the mark of a poseur or a noob.
A trip to a grocery store, convenience store, or gas station for the sole purpose of buying an excessive amount of junk food and sugary drinks, usually with friends. A chud run often happens late at night and ends with everyone involved hating themselves afterwards.