An accurately, but unintentionally placed punch below the belt line, with the aggressor usually being a child and the victim being a male, either the childs age or older.
The ideally placed zipper punch will render the victim helpless as he assumes the fetal position after immediately falling down. This is then promptly followed by any number of grunts, groans, tears, and sometimes laughter.
My daughter and I were roughhousing the other day and she leveled me with a Zipper Punch. And I almost passed out. Atta girl.