An extremely useless portion of your body unless you have a hottie who could sit on there comfortably and you could make some lovely memories, Yes, I'm talking about your thighs, your manly lap. The idea behind calling it "Upside Cheeks" is - Your but are cheeks which are down while you're sitting or laying down therefore, Your thighs/lap which is opposite to your downwards cheeks should be called as Upside Cheeks.
Pajama casual is a term used when working from home and preparing for a video conference. All business above the waist and your pajama pants down below.