It's not really an insult. It is more of a cute way of saying that you already know the answer to the question you are asking me. For instance, if I said, "why so many questions about American slang?" You might say, "...because I am learning English silly."

"That's why I know Chinese, silly." The word "silly" refers to someone foolish who shows a lack of common sense or judgement.

usually, this aslo mean"weak,stupid".... i am sad to hear that.but i don't think we are realy silly, we just need time to change and develop...