Jamming to your favorite music while wearing some extremely rock worthy/incredibly comfortable night clothes. The more the merrier, but also equally as fun when on your own. Add in some air guitar, or air drums, and your Pajammin session is complete.
I was really tired the other night but I just kept pajammin!
Person A- "Dude, you're still up?"
Person B- "Yeah, man. Once you start pajammin you can't stop!!"
Person A- "Good point. I'm coming over!"
Person B- "PAJAMMIN PARTY!"