黒歴史

Originated as an in-universe concept in the anime Turn A Gundam, referring to a sealed-away history of past tragic wars. The phrase caught on among Gundam fans on 2channel starting around February 2000 purely because it sounded good and fit the concept, then spread beyond Gundam fandom to the wider Japanese internet by roughly 2005, settling into its now-common colloquial meaning: an embarrassing past someone wants to forget or erase — a clean example of a term diffusing from fandom-specific jargon, through message boards, into everyday speech.