That is a difficult question but our 6 year old gave it a shot.
Here is the context. The family visited Toronto this weekend to get away from the Boston cold and get some Canadian cold instead. It was a fun trip although I have noticed that Canadians seemed to have missed out on the nouns and naming rights. What that means is that everything has to be qualified with 'Canadian' like Canadian bacon, Canadian geese, Canadian mounted police, Canadian dollars and Canadian Broadcasting Corporation.
Back to the story. We were out to dinner and the kids decided to rattle off 'Yo Mama' jokes. These are basically insults like "Yo Mama is so clumsy she trips over cordless phones" or "Yo Mama is so ugly the mirror refuses to reflect her image." There is even an iPhone app. The kids find this hilarious.
Anyhow, the eldest two were doing this and then the youngest wanted her turn. So she put up her hand to speak. At that precise moment, our waitress came by, saw the hand and asked "what can I get for you sweetie?"
I think you can tell what happened next. Our daughter looked at her straight on and said "Yo Mama is so fat she's stupid."
Suffice it to say, it is hard to give context to that but the waitress's face indicated a horror that was unimaginable. She was somewhat taken aback and speechless. Our laughter did diffuse the situation somewhat but I don't think there is any explaining precisely what happened. I think we will have to call it a Canadian insult and leave it at that.