well in sometimes 'fatty' refers to slow moving person. cowboy...

Michael is right. Also calling some one a 'fatty' is an insult meaning very fat and is used by primary children.

The correct phrase is "You are bananas" which does mean " you are crazy, mad or silly".

Hello Cappucci, "banana" is a slang term used to describe an Asian person. If someone is called a "top banana" it is another story. A comedy act in earlier days often included a part where one of the comedians would hit the others over the head with a soft object. The object was shaped like a yellow fruit: the banana. A top banana became a term to refer to the leading person in a comedy show. The funniest comedian is called the top banana. The next is second banana. And so on. Top banana still is used mainly in show business. Yet the expression also can be used to describe the top person in any area. When a person is called "fatty" not "faty" , it refers to a large person, a person greater than average size.

It would be 'you are *a* banana,' but that sounds really weird. I've never heard anyone use that as an insult for being silly. Sometimes we use 'banana' to describe Asians (Chinese, Japanese, Korean) that have lived in the US their whole lives and so act & think like white people (the inside of the banana), but look like Asians (the yellow outside of the banana). Some people consider this an insult but others just use it as a descriptive term for people with these characteristics. I'm not sure what faty means. I think it should be fatty. I would guess it relates to someone's weight. 'You're a fatty' could maybe be a young kid who was fat and slow and somewhat stupid.