A wild house (boat, library, club, etc.) party that requires you to come dressed up in your pajama's or sleep like attire some time usually during the night.
Like seen on House Party 2, when kid n play go to college. Please see this movie if you have any questions.
Denise: Hey what type of party is it?
Mark: You know, an old fashioned Pajama Jamie Jam!
Denise: Ohhh! I'll make sure to wear my new baggy shiry and Joe Boxer boxers!