Knowledge of a particular subject comprised primarily or completely of information gained from Wikipedia. Depending on the topic at hand, a Wiki-brain can be extensive and accurate or miniscule and fictitious.
One: How do you know so much about Metallica? You're not even a fan!
Two: I'm not, but I have a Wiki-brain about them.
One: You know that help you gave me on that essay last night? It was all wrong! I failed!
Two: Sorry, man. I warned you it was just a Wiki-brain.