A girl who you really want to sleep with but afterwards feel sort of depressed that you did. Normally requires vomiting or a gym session to clear your mind. Very similar to the feeling you get after you eat McDonalds.
Bro 1: last night i totally did some mcpoon, want to hit the gym after class so i can get over it?
Bro 2: sure bro, i normally vomit to get over mcpoon, you should try it.
McPoops are the shits you take right after eating McDonalds. One usually gets the McPoops after eating a McDonalds item that doesn't agree with you or after eatig healthy food for awhile and then going back to McDonalds.