The term 'magnum opiate' originated on the cover of the Principia Discordia and is now occaisionally used to describe other Discordian works. In its original context:
Principia Discordia
*OR*
How I Found Goddess
And What I Did To Her
When I Found Her
THE MAGNUM OPIATE OF MALACLYPSE THE YOUNGER
WHEREIN IS EXPLAINED
ABSOLUTELY EVERYTHING WORTH KNOWING
ABOUT ABSOLUTELY ANYTHING
The meaning of the term, like much of Discordianism, is multi-layered. The phrase is an intentional malapropism of 'magnum opus' and a riff on the famous (and apocryphal) quote from Karl Marx, "Religion is the opiate of the masses." One of the central ideas of Discordian philosophy is that all beliefs are to some extent illusory. The word 'opiate' evokes this idea, implying that by accepting the ideas within, the reader is deluding himself.
'Magnum opiate' is also one of many drug references in the Principia. Take, for instance, the footnote concerning the Golden Apple of legend: "There is historic disagreement concerning whether this apple was of metalic
gold or acapulco."
Of course, it is entirely possible that the whole thing was meant as a joke.