When one wets a finger generally the pinky usually by covering in saliva then inserts into somebodys earhole and spins eraticly causing overall discomfort in order to wreak havoc or humiliate.
A wet willy is a prank that involves wiping a stick of butter all over your dick and then sticking your dick into an unsuspecting victims ear when they are not looking. This prank was used commonly during the Great Depression Era to lift the spirits of out of work Americans and was recently revived when demonstrated in the 1980's sitcom "The Facts of Life", when Phil Drummond (of Different Strokes) came to visit the girls at their school.
"Natalie was so flattered by the attention she was recieving, she not only
forgave Phil for the WET WILLY, she also forgot she was so fat for a few