Si Way, Jóse

I was in my first homestay one day, eating lunch with Maria. My host sister Cony walked into the kitchen, searching for something to eat. Maria began to give her a hard time about her weight, which was ridiculous because Cony is quite slender and pretty. Here´s the conversation, translated from Spanish:

Maria: Oh, gorda, why do you want to eat something? You´re getting fat!
(Cony just ignored her mother´s uncalled for nagging.)
Maria: Hayden, don´t you agree with me that she´s getting fat?
Me: No, no I don´t.
Cony: Of course she is, just look at her.
Me: No way, Jóse.
Maria: Huh?
Me: In my country when we disagree absolutamente, we sometimes say the phrase no way, Jóse. ¨No way¨ means claro que no and ¨Jóse¨ is of course just a name.
Maria: Oh, I get it. Si way, she´s getting fat.
(Cony rolled her eyes and I just laughed.)