1
general::
A past event, as in She's talking about her sea voyage, but that's ancient history, or And then there was his divorce, but you don't want to hear ancient history. This hyperbolic idiom transfers the field of ancient history to a much-repeated tale.
American Heritage Idioms
2
general::
Phrase(s): ancient history
Fig. someone or something from so long ago as to be completely forgotten or no longer important, as a former relationship. • Bob? I never think about Bob anymore. He’s ancient history. • His interest in joining the army is now ancient history.
McGrawhill's American Idioms And Phrasal Verbs