"Once a girl" 是指曾经是一个女孩的意思。这是一个普通的表达方式，没有特殊的来源。

以下是一些例句：

She was once a girl with big dreams and aspirations, but now she's just going through the motions.
（她曾经是一个有着大梦想和雄心壮志的女孩，但现在她只是在敷衍度日。）

Once a girl with bright eyes and a sunny disposition, she's now a bitter woman who's lost all hope.
（她曾经是一个眼睛明亮、性情阳光的女孩，但现在她是一个怨恨的女人，失去了所有的希望。）

I can't bepeve how much she's changed since we were kids. She was once a girl who loved to laugh and play, and now she's just so serious all the time.
（我难以置信她自我们儿时起发生了多大的变化。她曾经是一个喜欢笑和玩的女孩，而现在她一直这么严肃。）