knuckle tuck
A knuckle tuck is when one curls up one's finger, tucks said finger into the receiver's anus, and continues to add more and more fingers.
Oh my God, Becky, Bryson knuckle tucked my camel toe for an hour straight.
Knuckle Tucking
When a male tucks his weenie between his legs.
During a scene in "Silence of the Lambs", Buffalo Bill is "Knuckle Tucking" infront of the mirror.