Apparently, there is something called a 'wet willie,' and its not what immediately sprang to my mind when I read the phrase. Prepare to be educated:

According to court records, Louis Carlos Perales, 38, picked up an additional charge of assault and battery on a police officer after he put his finger in his mouth and then into an officer’s ear. That, my friends, is a wet willie. We live and learn.

Perales had been arrested for threats to kill and public intoxication at the time of the incident. Well, that makes sense, how intoxicated does one need to be to inflict a wet willie on an officer of the law?

Examiner-Enterprise