Its somewhere between free period and philosophy. In other words, IB didn't want you to possibly have a free period for half the year, no way, they wanted to fill in that space with another class with a name equivalent to "bull shit." Sometimes for "fun", teachers assign the reading of Sophies World.
Today in Theory of Knowledge, I BSed an 1,500 word paper about math as a way of knowing in 25 minutes!
In VTuber culture, Forbidden Knowledge refers to the past life and identity of a VTuber talent. The reason it's called this way, is because the voices/talents of VTubers are meant to be anonymous beyond their VTuber persona.
"Hey, I just found out this VTuber's Forbidden Knowledge. No wonder she reached 4 million subscribers so easily."
An expression meaning that whatever you say is based off of whatever information you know. Can sometimes be used to say you're just assuming something.