Derived from "Mongoloid" meaning one who suffers from Downs Syndrome and "Retard" a term given to either slow learners or just general morons, this word is used as an insult to people who do something spasticated.
"John you managed to set fire to your own fireproof boxers you mongtard!"
Quite simply a mixture of a minger (someone who fell out of the butt ugly tree and hit the sphincter on the way down) and a fucktard (someone who isn't retarded, but who is so stupid/annoying/idiotic, that they sure do appear that way.)
Not only was that barmaid as fat an ugly as a hippo, she couldn't even subtract 4 from 10. What a mingtard!