Munging implies destruction -- to make large-scale and irrevocable changes to a file and to destroy it. Hence, in the early text-adventure game Zork, also known as Dungeon, the user could mung an object and thereby destroy it (making it impossible to finish the game if the object was an important item). A person who Dealing with vandalism a Wiki page would not be munging that page because the changes could be reversed.
Mung may have been created from the Lowland Scots word 'munge', meaning to imperfectly transform or, later, to munch up into a mess.
Alternatively, according to Charles Mackay's book, Lost Beauties of the English Language, published in July 18th 1874, there was an early American term, "mung news", which meant "false news". This was because, according to Mackay, mung is an obsolete past participle of mingle, and so mung news was news that was so mingled it was impossible to determine what was true and what was not. This may be the origin of Mung.
In some circles 'mung' is used as a word meaning foul material covering on a surface, i.e., "you've got some mung in between your teeth" or "you left your mung on my table." Military mechanics loosely use the term to refer to a combination of axle grease, mud, and dead things that were crushed under the equipment, and anything else that is generally left to be sprayed off by the lowest ranking shop worker. This concept has spawned cast-offs, implying that anything that is supremely disgusting can be said to be "mung".
Carrie Fisher, who played Princess Leia in the original Star Wars trilogy, has referred to the substance that oozed from the mouth of the Jabba the Hutt puppet (as seen in the film Return of the Jedi) as mung.
In the April 1977 issue of Creem, Rick Johnson wrote: "...they're not any good, they're not so bad they're good, they're not anything. Their only hope of crawling out of the mung heap is..." in a scathing review of the all-girl rock band The Runaways. He also used the term "mung heap" to refer to Black Sabbath's songs in the October 1979 issue of Creem.
This article is based in part on one in the jargon file; the jargon file is in the public domain.