SPISPOPD, part of the cheat code "idspispopd", is an abbreviation for Smashing Pumpkins Into Small Piles Of Putrid Debris.
Its origin can be traced back to the following Usenet post by Eli S. Bingham in early December, 1993, from the comp.sys.ibm.pc.games.action newsgroup:
Listen up, ID Software! Next time you have an impending release of a much anticipated game, make sure its name is not so cool-sounding as DOOM and much longer to eliminate all of the casual "Where can I get xxx" posts. How about "Smashing Pumpkins Into Small Piles Of Putrid Debris" for the next game?
This initial quip was carried on, and a parody of The Doom FAQ was written called the SPISPOPD FAQ. The FAQ described a 3D game that ran on a computer with minimal specifications.
A second reference apparently sponsored by id Software appeared in the first Doom novel, wherein one of the two major characters says, "Smashing pumpkins into small pieces of putrid debris," after killing a Cacodemon (which they dubbed a "pumpkin").
Two computer games were created based on the joke. Smashing Pumpkins into Small Piles of Putrid Debris is a 2D action game from Jamul Software (later Hamumu Software) in which the player collects candles and smashes pumpkins with a red hammer. It was reportedly developed in 48 hours to capitalize on the joke. Hamumu Software later created a similar game called Amazin' SPISPOPD.
Simtex's 1994 game Master of Magic included a magical item called Idspispopd.