A person you make up to get out of a situation or add to a story to make it sound more realistic. Comes from Season 2 Episode 9 of Community, when Jeff Winger, (Played by Joel McHale), made up a fake professor to get a free community college credit. The person being made up usually ends up getting called out, and people realize that the guy who made him up is a huge liar.
Person 1: Dude, I was at this party and this chick Jessica totally walked up and started making out with me! You can ask my friend John if you don't believe me!
Person 2: I think this "John" is a total Professor Professorson.