What does the joke "one eunuch is missing" mean? My friend told me to go back and think for myself. Thank you!

First of all, the original text of this joke should be that a man wants to tell a joke. He said, "Once upon a time there was a eunuch." Then a eunuch next to him asked the man, "What's next?" So the man replied, "There is nothing down there."

The eunuch asked, "What's the next step?" I asked the man what the bottom of his joke was, and the man replied that there was nothing below. In fact, he said that the eunuch's "bottom" (that is, JJ) was gone, that is, he laughed at the eunuch for not having JJ.