Loose, baggy, and often old pants without buttons, zippers, or belt loops. They are worn to be easily taken off, or at least to allow easyfingering/hand jobs.
Babe, your mom is downstairs. Go put on some shag pants and let's "watch" a movie.
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.