A shortened version of the Shakespearian insult 'fish monger' from Hamlet directed at Polonius. There is debate to what the original meaning was but it usually donates to the nature of the fishmongers of the time who were money grabbing, corrupt, stereotypically simple and of low moral fiber. So mong generally refers to a despicable and or stupid person.
(Disclaimer: this is not a stab at anybody associated with the fishing industry. it is just an interpretation of another mans writing and opinions)
"you complete and utter Mong"
"You can't stick that there, you mong!"