When someone is jacking it so hard that it looks like they are boxing the blanket
"I wanna be a blanket boxer when I grow up", said Johnny.
"I really need to stop leaving you home when daddy and his friends are drinking" sighed his mother
"You would never guess what Matt was doin when I walked into the room." "Lemme guess, he was blanket boxing." "How did you know?" "Cuz he loves men"