Good one. I thought of that, too. But the bit was this: His wife was trying to describe what childbirth was like and she told Bill "Try to imagine pulling your lower lip out and then wrapping it over the top of your head."
I remember the bit you described. However, he also talked about getting a filling and how his lower lip was falling into his lap after he got the Novocaine. Remember, "mybe fabace... mybe fabace..."? [Best transliteration I can manage.]