Laughing so hard in a crowd that you can feel it in your ass. It is best felt when sitting down than standing.
When I was seeing a show called the Twilight Zone. The production was so comical. It had to be told. I wrote a text to a friends saying " I rectal laughing so hard that night"What I wanted to say is that recall laughing. This term is used in my friend circle now. People with often say Rectal laughing. This just makes me want to laugh, laugh, and laugh some more.