Mongo is short for Mongole/Mongölchen which is a very, very rude word to refer to someone who suffers from Down's Syndrome.

Please don't ever use it!

What is the context?

As a rude equivalent you might use "Shut up, retard.' but this is also very rude. You might hear kids talking to each other like this though or very immature adults.

@ woodelf: I would agree that "Shut up, retard!" is a good equivalent to "Schnauze, Mongo!". But as DeepThought stated, it derives from mongoloid or Mongoloismus and those terms are not used anymore (to be politically correct).

But if it comes to insults, idiot, halfwit and retard originally had the connotation of "not right in the head" or "mentally disabled", while nowadays we may not think about that when using those terms.

Im BE war die Beleidigung "Mong" vor 10 oder 12 Jahren geläufig.

The German word so called "Mongo" means, if you spectate it from the view of parents and politicans that you want to affront somebody with down syndrom and from their view it's a very rude word. But nowadays it's often used by teenagers who doesn't really mean it as an insult. Most teenagers use it as an alternate for (just an example):"Your dumb, dude!" The way you accent the word is really important because it's the accent which decides if it's an insult or just an alternate for my example. (In a more or less friendly way)

Remember that what I wrote isn't or has not to be my personal opinion!