one’s embarrassing secret/past (that one wants to forget and hide from others); skeleton in the closet
It is said to originate from the Japanese popular anime “Turn A Gundam”. In this anime, there is an era called “Universal Century” aka 黒歴史 (“black history”), which witnessed a great number of space wars.
It’s relatively a new slang word and is mainly used on the internet.
The skeleton in my closet is that I used to be dating him.
It is absolutely my embarrassing secret that I used to upload my sh*tty rap videos on YouTube when I was a junior-high-school student.
A: Hey dude, back in high school you asked out ten girl classmates in one day and got rejected by all of them, right? lol
B: Hey stop it, don’t remind me of my embarrassing memory