my ass!

Why do people say 'my ass!' when they want to criticise/ridicule something? What does an 'ass' have to do with anything?


Person1: That was a great film!
Person2: A great film my ass!

Person1: Pizza is good for your health.
Person2: Good for your health my ass!

It's interesting the role of the word "ass" in the English language. For example "kiss my ass" is used as an insult, when in other languages this may be considered as the begining of a relationship.

<<when in other languages this may be considered as the begining of a relationship.>>

Really? Can you give an example of such a language?

Spanish