When something is especially dull, it makes us feel like our brains have stopped working — it's mind-numbing. If you've had to listen to a lo-o-o-ong, bo-o-o-oring lecture, you already understand what mind-numbing means.
Mind-numbing describes something that is so boring and uninteresting that it turns your brain into useless mush. The word was first recorded in 1898, but of course, people were experiencing boring work and dull teachers for a very long time before that. Even the Ancient Greeks must have been bored every now and then — but, unlike you, they didn't have this word to describe just how bored they were.