An extention of the popular, "pool of knowledge" that basically means someone is so smart and has such a large information base they surpass just being a pool of knowledge, they can miraculously pull things out of their ass, and hence are a cesspool of knowledge.
Could be taken as a compliment or an insult--i.e. someone that is full of shit.
You're so smart you're like a cesspool of knowledge!
#knowledgeable#smart#educated#pull things out of your ass#full of shit#awesome#full of it