/muhng/ (MIT, 1960) Mash Until No Good.
Sometime after that the derivation from the recursive acronym "Mung Until No Good" became standard. 1. To make changes to a file, especially large-scale and irrevocable changes.
2. To destroy, usually accidentally, occasionally maliciously. The system only mungs things maliciously; this is a consequence of Finagle's Law.
Reports from Usenet suggest that the pronunciation /muhnj/ is now usual in speech, but the spelling "mung" is still common in program comments (compare the widespread confusion over the proper spelling of kluge).
3. The kind of beans of which the sprouts are used in Chinese food. (That's their real name! Mung beans! Really!)
Like many early hacker terms, this one seems to have originated at TMRC; it was already in use there in 1958. Peter Samson (compiler of the original TMRC lexicon) thinks it may originally have been onomatopoeic for the sound of a relay spring (contact) being twanged. However, it is known that during the World Wars, "mung" was army slang for the ersatz creamed chipped beef better known as "SOS".