Something or someone from the past, esp an old romance: Max is ancient history
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with ancient history
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.