VERB

1. THE ACT OF TURNING THE POUND (FIST BUMP) UPSIDE DOWN

2. A DIFFERENT FORM OF FISTING WHERE THE FIST IS INSERTED (IN A PALM UP FASHION) INTO THE VAGINA OR ANAL CAVITY
WHEN TWO PEOPLE GREET EACH OTHER WITH A FIST BUMP, THEN TURN THE FIST BUMP UPSIDE DOWN AT THE SAME TIME, THAT IS CALLED AN UNDERKNUCKLE.
by Segue Taylor March 05, 2020
Get the UNDERKNUCKLE neck gaiter and mug.