Mong is short for 'mongoloid', which was originally an anachronistic term for a Down's Syndrome sufferer. The modern Mong however is a total fuckwit, who deserves nothing less than complete humiliation for their idiocy. That includes all those dope smokers too, who relate being 'monged' to being chilled and mellow. Its because when you're stoned you act like a fucking retard, you fucking spazzes!
"You spilt yer pint all over my strides, you FUCKING MONG! I'LL KICK YER FUCKING CUNT IN!"