The speaker opens with a blunt, slightly shocking claim: the best part of a woman's body is the spot where a tail once was. Then she catches herself, wondering if the thought is "atavistic," a word meaning a throwback to something primitive. The tone is wry and self-mocking, half joking and half uneasy about what her own desire says about her.
The second stanza shifts to a quieter memory. Her father found notebooks full of words she had learnt but never used, and "atavistic" was one of them. The detail suggests a private, curious young person collecting language she wasn't yet ready to speak aloud.
The last stanza turns to him. He was "intelligent but conflicted," and the speaker thinks it will take another generation for his ideas to "filter through." The notebooks might finally be useful then. The mood is thoughtful and tender, with a little frustration. The poem seems to be about inheritance: what we take from our parents, what we outgrow, and what we hold onto until the world is ready for it.