Self-licensing is the unconscious1 tendency to allow yourself to do something bad after you have done something good.
Imagining yourself as "doing good" can sometimes lead to bad things. You eat more at Thanksgiving because you went to the gym in the morning. The "good" action somehow licenses2 the "bad" action, because you have a self-image as a healthy person.
It's what many diet experts say may be behind why so many of us fall off the diet rails on our "cheat days."