Answer: His neck
While in Jade's apartment, the police barged in and arrested Stu, Phil, and Alan. They informed the three men that the previous night, the guys had apparently left their car in the middle of Las Vegas Boulevard and then stole their police car. So to make amends, the guys had to participate in a demonstration for little kids. The police officer was telling the crowd of children that there were two ways to use a stun gun. He said, "Up close and personal," and then stunned Stu in the neck. After Stu went down, a little girl stunned him "below the belt", so to speak, and Alan was stunned in the head and neck.