A childhood belief about swearing

When I was little, I saw my brother giving the finger to this kid across the street. He was standing in our courtyard in front of the house. I asked him what it meant, and he refused to tell me. So I gave the guy the finger,too. My brother was so mad at me and he refused to tell me what it was all about. Then I went inside and asked Mom what it meant and showed her the finger. She asked me where I had learned it. I told her from my brother. Her face became red (from her trying not to laugh), and she said, "that is a very bad word, and you should not say that again".