A clever combination of "that's what she said" and "your mom!". It was created to avoid the argument that "that's what she said" creates confusion because "she" is nobody while adding the punch of "your mom" jokes to it.

The phrase "last night" may be added to the end if extra pwnage is needed.

When you eat dirt for the first time and leave your family to build a shack in the woods

A mix of that's what she said and your mom.

Basically means the same thing as that's what she said, except you use your mom instead.