Either a person who is a snitch that is disliked by many, talking about people behind their backs and just generally being really sneaky, or someone who is considered silly or weird, but is called a rat affectionately by their friends
A term used to describe another person as in the way people call each other dogs. It came from the Derby Lane dog track when a bum bet on a dog and was so frustrated that he called the dog that lost a rat! We overheard this and every time we bet a dog that lost we called him a rat. Then we started calling each other rats for betting on rat dogs. The term rat can be used as a compliment and as an insult. It all depends on how you use the term.
I can't believe this rat lost all his money at the casino. He's such a rat!