I'm sorry, I've never heard this expression before! Where did you see or hear it? Rats eat garbage, so maybe "rat dinner" is garbage or food somebody put in the garbage. I guess "rat dinner" could also be a rat that a person eats for dinner ...

头次用，没搞明白

你类别写错了吧，这应该是英语学习的，你写成汉语的了。