When you finger your girlfriend and immediately give someone close to her a wet willy with the same finger.
For girls: when you jack your boyfriend off and get cum on the tip of your finger and immediately give someone close to him a wet willy with the same finger.
Dude, he totally took biological warfare to a new level when he gave his girlfriend's brother a pickled willy.
Not only did Justin get Rodney with a pickled willy, he got the dog too!
To lick your finger, insert it in someones anus, and twist it around in the fashion of a Wet Willy.
Dude: "I was getting my girlfriend doggie style when I gave her a stink willy!"
Bro: "You licked your finger and stuck it in her butt?"
Dude: "Totally!"
Bro: "Bro Grabs!"