kiss one's ass直译过来就是：“亲某人的屁股”，这个短语的正确意思是：“巴结某人”。因此，当美国人说"But you can kiss my ass."时，他/她要表达的意思就是:"But you can try to curry favor with me."、"You can try your best to please me."。
Shirley: I'm so sorry that I've made a big mistake in this project. Please don't tell the manager about this.
Colleague: The manager told me to report everything to him. But you can kiss my ass.
①It's not convenient to help you right now.
②I'm also busy with my own project.
③I don't have to do you a favor.
④I'm not so familiar with your project.
curry favor with 奉承讨好
He brought her some flowers, hoping to curry favor with her.
He never tries to curry favor with those over him.
Jack did everything to curry favor with his superiors in the hope of getting promoted.