When you take a woman's nipples and place one on each eye like you are looking through binoculars and try to adjust the focus. This is normally done in a strip club and is better than motorboating.
This is horrible abuse to do to a person. You rub your knuckle against either your own, your victim's, or a third parties butthole then proceed to rub your "dirty knuckle" against your victim's mouth.
Tiffany: "Danny, if you don't stop making fun of my nips, I'm gonna give you the dirty knuckler!"
Danny: "NAY!"
Taking your fists pressing them together where both tips of your knuckles are pointing out and creates a V shape. Another person does the same and you press your knuckles together making a Diamond and insert the penis and proceed to have sex with it