SPISPOPD became so well known that it reached Doom's coders, and one of them snuck in the code - IDSPISPOPD - at the last minute. Sadly, it was later changed to IDCLIP for the sequel, Doom 2. Ah, well. Cest la vie.
The qualtiy of being able to, or in fact to pass through something without expected obstruction.
Origin: No clip cheat code from the computer game DOOM (allowed you to walk through normally solid walls)
Generally never spoken aloud.