dark past; cringeworthy history; something one wants to forget
Internet slang for embarrassing or shameful past events, often from one's youth, that one wishes to erase or pretend never happened. Commonly used self-deprecatingly.
The poems I wrote in middle school are totally a dark past I want to forget.
I found my old blog and ended up digging up my cringeworthy history.
The term originated in the anime series 『∀ガンダム』, where 黒歴史 denoted a concealed history of past wars. It later spread as slang for an embarrassing past one would prefer to forget.