I’m not sure if wet-willies are an American invention, but in the case that it is not known:
“A wet-willy is achieved when a tru mofo lubes his index finger with saliva and then proceeds to stick the moiztened phalange in an unsuspecting foo’s ear. The phallic overtones of the moist penetrating finger suggeztz without a shodow of a doubt that the translation of the British “willy” was somehow ingeniouzly preserved. 
”
If don’t know, now you know.