Going back to my dusty old memories of the huge waste of time that junior college was...
This used to be a technical term, much like mentally retarded. It comes from psychology, Sigmund Freud used it as a descriptive term for infants who were fastidious, and didn't play with their poop. As opposed to the mentally retarded individuals who played with their own poop, referred to as anal expulsive.
The story from the professor was that some infants/children got caught "playing in the mud," so to speak, got such a thrashing that they learned to retain their ****...
