Back in the days of slaves there was no bug zappers. In order to keep the bugs away slave owners would lather jam onto a slaves face and make them walk beside them. The bugs would naturally be attracted to the jam leaving the slave owner bug free. Calling slaves jam faces was the same as calling them nigger.

A condition that makes it look like somebody has slothered jam all around your mouth. Requires one to smear vasiline all over his face.