Short for mongrel. A term usually used to describe someone who goes off with your ex boyfriend the minute you split up. At the same time she is also a 0 rating on the stunning scale and can often look like a ugly dog or horse. The mong also tends to buzz off presents you bought the ex, unknown to her.
How did you break up?
Oh he went off with a mong.
Yes, apparently a dog was let loose out of the kennels.