It is short for "Mongol", the original word for someone with Down's Syndrome. Such people generally have a very low IQ and a limited ability to look after themselves. It is the effect of a chromosomal abnormality and is common in the offspring of older mothers. The eyes of a sufferer look rather like the eyes of an East Asian. As the report below notes, the term "Mongol" has so dropped out of use that many young people would not know it as a description of Down's Syndrome -- so the claim that it is derogatory of such people is weak. But it is certainly cruel and in bad taste to mock the disabled.
Ricky Gervais has sparked fury among the disabled community with his repeated references to 'mongs' on Twitter.
People with disabilities and their families have been outraged by his persistent use of the word, which they say is a derogatory term for somebody with Down's Syndrome.
The comedian posted a series of tweets including phrases such as 'Good monging everyone' and 'two mongs don't make a right', along with pictures of himself making silly faces.
But the creator of The Office insisted that the meaning of the word had changed, and said the angry reaction was equivalent to being offended by the word 'idiot' as an insult for disabled people.
However, Down's Syndrome campaigners and disability rights groups attacked him for refusing to back down....
'His fanbase are now using the word to attack me and others who have criticised him on Twitter. A new generation who didn't even know this word are using it abusively, and that's bullying.'
Interestingly, there is an old Australian slang term also meaning a really stupid person: "Nong". I have no idea of its origin but I guess it could be a version of "Mong". Brits and Australians tend to know one another well so I wonder if the guy above had heard the Australian term and just got it a bit wrong.