Another way of saying that you are giving someone the middle finger. When you wear mittens you may conveniently flip the bird at someone without their knowledge. An actual mitten or a picture of a mitten will also convey this.
1. "Why were you waving at that cop?"
"I wasn't waving, I was giving him the mitten."
2. "Wait a minute, there's only one mitten in this box. Did you lose one or are you giving me the mitten?"