Possible Clues:

• Prank performed on someone in a headlock
• Jocular headlock accompaniment
• Playful knuckle rub
• Head scratcher?
• Knuckle-headed antic?
• Knuckle-headed gesture?
• Knuckle to the head
• Playful knuckle-rub
• Knuckle rub on the head
• Headlock accompanier, often