The Reverse of saying "Your Mom" as a joke. it can be used in the same way. People use it if you don't want to hurt someone feeling about there mom. A good way to laugh at your self. Most of the time when used it doesn't make sense but is still funny. This is best see in used in the Cartoon network TV Show "Regular Show" by Muscle Man.
Person 1- Hey I'm gonna do my homework now
Person 2- You know who else is gonna do there homework now? My mom!