On that day, she had written him a big long letter and told him that it was all off, for she didn't love him any longer.

off 在这里是副词用法，表示"结束、终止"的意思，常用于表示关系或约定的终止。在这个句子中，it was all off 表示"一切都结束了"，特指恋爱关系的终结。