Why was the book titled The Pigman?

The first sentence of chapter one alerts readers to the book's title.

Now, I don’t like school, which you might say is one of the factors that got us involved with this old guy we nicknamed the Pigman.

John tells his readers that at some point in the book, he is going to tell us about some old guy that he and Lorraine gave a very unflattering nickname too.
It turns out that this man is named Mr. Pignati. He turns out to be an amazing influence on both John and Lorraine, and with that last name, it makes sense that John and Lorraine might nickname him "The Pigman." But wait. It turns out that Mr. Pignati has a collection of ceramic pigs, so his nickname really makes sense. What's great about the nickname and the title is that is sounds completely unflattering; however, as John and Lorraine come to love Mr. Pignati more and more, the nickname winds up being a term of deep endearment. The book is titled the way that it is because the book is John and Lorraine's "memorial epic" that is dedicated to their dear friend Mr. Pignati -- The Pigman.

She’s got very interesting green eyes that scan like nervous radar—that is they used to until the Pigman died. Ever since then her eyes have become absolutely still, except when we work on this memorial epic.