The narrator no longer feels safe because her life is changing.... the definition of "girl" is encroaching on her world.... telling her what she should be, how she should act. The realization of the constraints placed on women make the narrator feel as if she if losing her freedom.
This winter also I began to hear a great deal more on the theme my mother had sounded when she had been talking in front of the barn. I no longer felt safe. It seemed that in the minds of the people around me there was a steady undercurrent of thought, not to be deflected, on this one subject. The word girl had formerly seemed to me innocent and unburdened like the word child; now it appeared that it was no such thing. A girl was not, as I had supposed, simply what I was; it was what I had to become. It was a definition, always touched with emphasis, with reproach and disappointment. Also it was a joke on me. Once Laird and I were fighting, and for the first time ever I had to use all my strength against him; even so, he caught and pinned my arm for a moment, really hurting me. Henry saw this, and laughed, saying, "Oh, that there Laird’s gonna show you, one of these days!" Laird was getting a lot bigger. But I was getting bigger too.
My grandmother came to stay with us for a few weeks and I heard other things. "Girls don't slam doors like that." "Girls keep their knees together when they sit down." And worse still, when I asked some questions, "That's none of girls’ business." I continued to slam the doors and sit as awkwardly as possible, thinking that by such measures I kept myself free.