[in 1960 at MIT, “Mash Until No Good”; sometime after that the derivation from the recursive acronym “Mung Until No Good” became standard; but see munge]
1. To make changes to a file, esp. large-scale and irrevocable changes. See BLT.
2. To destroy, usually accidentally, occasionally maliciously. The system only mungs things maliciously; this is a consequence of Finagle's Law. See scribble, mangle, trash, nuke. 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. In the wake of the spam epidemics of the 1990s, mung is now commonly used to describe the act of modifying an email address in a sig block in a way that human beings can readily reverse but that will fool an address harvester. Example: johnNOSPAMsmith@isp.net.
4. The kind of beans the sprouts of which 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 U.S.: army slang for the ersatz creamed chipped beef better known as ‘SOS’, and it seems quite likely that the word in fact goes back to Scots-dialect munge.
Charles Mackay's 1874 book Lost Beauties of the English Language defined “mung” as follows: “Preterite of ming, to ming or mingle; when the substantive meaning of mingled food of bread, potatoes, etc. thrown to poultry. In America, ‘mung news’ is a common expression applied to false news, but probably having its derivation from mingled (or mung) news, in which the true and the false are so mixed up together that it is impossible to distinguish one from another.”