In football, when a player hits another player so hard, that the player who was hit has snot literally knocked out of his nose. More common in the winter when people tend to have colds. Sometimes called a "slobber knocker".
Wide recievers fear going over the middle, because they're afraid they'll get a snot knocker.
A child, more specifically a child with a cold/runny nose.
I think I caught a cold from that little snot knocker.