In UK slang, to mong means to vegetate: To be without doing, and the state of mind that comes with this. Tiredness, television, and weed-smoking are all leading causes of monging out, which is to say being subsumed by the mong, and just sort of sitting there.
The origin of the word is unclear; either it is onomatopoeia for the sound sound of an inactive brain, or it derives from mongol, an offensive term for people with Down's syndrome, or possibly mongrel.
Also: I learn from Chambers dictionary that 'mong' (pronounced 'mung') can mean 'a mixture or crowd', hence among. It may also be what a monger does, as in a fishmonger or an ironmonger, but it's hard to be sure.