Means Something that is disgusting, broken, or both.
Usage "Aww, man, that's proper mung""It's mung, Steve saw to that"
Throw together a bit of common-or-garden computer geekery and an urban myth so revolting we're sure you don't want to hear its full extent while sipping your breaktime beverage, and you have a catch-all word for almost everything that can disappoint a humble Yoof. Packed-lunch sandwiches filled with something indescribably yellow-green? That's mung. Stuck with a computer in ICT that's slower than the Tube in rush hour? That's mung.
The "broken" part comes from programming slang, and the acronym MUNG, meaning Modified Until No Good, when a piece of software has had so many fixes and patches added that it no longer works. The "disgusting" part involves a very specific act of necrophilia. If you really want to know (you do, don't you?), a quick Google search will throw it up for you (in more ways than one). Safe to say, you know something is suitably vile when teenage boys who refuse to shower more than once a week coin it as their go-to word for everything putrid and messed up.