Doing something that normally has an agreement between 2 parties with a secret 3rd party. Unbeknownst to one of the 2 original parties involved. This can include under the table business dealings to gain an increase in funds without a person who you have an agreement with knowing so they never ask for a cut of the profits. Also can be used as a synonym of cheating on one's boyfriend/girlfriend/spouse.
I know this one guy who is always doing side deals and thinking that he will get away with it well I know about it and I better get my cut or he will not have a good day at all.