rat

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"