Things the Japanese Do: Think "silly" is a cuss word

Recently my brother has taken to chiding my mother and father for using the word "stupid." He says it's a bad word that we shouldn't use. Instead, we should substitute the word "silly" for when we'd normally opt for "stupid."

They wouldn't air it in Japan if it was "Two Silly Dogs."

As a native English speaker, I can tell you with all authority that this is perfectly fine in most situations. "Stupid" can have a very negative connotation, especially when combined with other 4-letter words. However, "silly" has no weight to it. For example, let me present two phrases:

1) You are so stupid.
2) You are so silly.

The first phrase is quite legitimate and I'm sure most native speakers have heard it before or even hurled it at someone themselves. You would be calling someone an idiot and thems is definitely fightin' words. I can almost guarantee that you will never hear the second one in any sort of serious confrontation or argument, because it is simply too fluffy. It's more of a light comment you'd make after seeing a baby animal or small child do something cute, or a phrase you could use if you were a writer for Monty Python.

In the completely logical country of Japan, this is not the case. In my personal experience, "silly" is considered a heinous taboo word that should never be uttered.

In one of my classes this past spring, a young child made a small mistake and I said, "Oh [dear child whose mother I loathe], don't be silly!" The mother later complained to my manager that I had called her child "silly." I have written about my work situation before, so I will just say that we did not come to an understanding about the word "silly" as native speakers see it. Because you know, we should disregard whatever the native speaker says about their own language in regard to how a non-native speaker misinterprets it.

My grandmother student has also said that "silly" is a terrible word to use and has warned us against using it with her granddaughter. I should really bring it up in class one day and assure her that there are far more filthy terms at a native speaker's disposal when dealing with children. I did successfully teach her the term "brat" and she even used it in class to describe said granddaughter.

Japanese people do have a strange affinity for the word "crazy." I realize we use this term to describe a variety of things, but it can quickly become rude when describing people. I hear a lot of, "My kid is so crazy!" "S/he is so crazy!" and they don't mean to be offensive but I know to many people it would be. Yet no one in Japan would ever talk about mental health issues or people who are actually foaming at the mouth and rambling about how their best friend Zombie Satan wears a fedora on Tuesdays, so trying to explain things like that under most circumstances would be useless.

I think I should make a list of words to use instead of "silly." If I can't use silly, perhaps "fuckwit" is a better substitution. Suggestions welcome.