An insult, mainly towards a fat or obese person, male or female, who refuses to do anything about their wieght and just keeps on eating fatty and greasy foods, and they frequently use excuses that such as "I will always be that way" or "God made me this way". Other grease traps wallow in their own misery at the fact that they are so fat and sit in front of the T.V. eating a bag of Doritos and drinking a mcDonalds milkshake while watching Maury or Oprah when they could be working out instead of eating so much grease. This is why they are called grease traps, because they are aware of the problem, and will not make healthy chioces in order to change the situation.
Girl #1: Oh that Tim guy eats way too much greasy foods. The sad thing is that he refuses to do a thing about it. He just keeps eating and eating.
Girl #2: He has become such a grease trap.
Girl #1: Yeah tell me about it. The fryers at McDonalds has less grease in their traps then he does.