What, or who, exactly, is the Pigman? Does it mean "a swineherd" (someone who herds pigs) as the dictionary states? Or a 2008 low-budget horror film? (It's not related to the book, by the way.) We find out fairly quickly; in the first sentence, John refers to "this old guy we nicknamed the Pigman" (1).
But why name the book after a pet name John and Lorraine give to Mr. Pignati, the Pigman? Well, because at the end of the book, when Mr. Pignati dies, John and Lorrain agree to write a "memorial epic" about their friendship with Mr. Pignati. The story that we're reading is supposed to be their "memorial epic," dedicated to the memory of their friend. For more on Mr. Pignati, check out our discussion of him in "Characters."