A rat is basically the new term for "burn out" or "loser". It is someone who doesn't value hygiene or any tidiness in their lives at all for that matter. Someone who wastes their time making poor decisions like regularly engaging in illegal activity such as drug use or theft.
one dude: "damn, look at that guys hair, it's go greasy"
other dude: "What a lazy bum. I heard he cheated off some girl's chemistry test the other day"
first dude: "wow, he's a total rat"
Rat means to be friends with someone or hate someone
Hey rat or your such a mole rat/rug rat
The most beautiful thing in the world!!! one day you are walking across the street you see a rat the rat is going into a sewer you decide to follow the rat the rat has led you to the cult of rats you see a rat get on stage with a sombrero his name is Pablito the rat
And then you see your best friend who keeps on claiming they don’t have a coat made of rats