TL;DR- Slang for something as bad as 576(AD), when a plague killed 20% of the world, and the sun was blocked out because of a volcano.
Used to describe something as beyond terrible. As context, the year 576AD is seen by many historians as the worst year in recorded history. This is due to a plague killing 40 million people (around 20% of the global population) and a volcano that blocked out the sun for most of Europe, causing famines on top of the plague. Therefore, this is why using 576AD as a slang term makes sense, as an event devastating to one person is comparable as an event devastating to all of humanity.
Bob: Oh no I have lost my job, Iām about to beevicted, and my house got robbed on top of that.
Jim: Sorry Bob! That is totally 576!