When a person is unaware that they are showing their true thoughts, feelings or emotions on their face, revealing to everyone around them their actual opinion on the matter. This often occurs in situations where the person's facial expression creates an awkward moment for the person/persons around them.
Friend: "Yo i can tell you were not impressed with her outfit."
Zigger: "Oh Crap! Was i zigging again. I bet everyone in the room knows what i was thinking now."