Big Brain was the evil overlord supercomputer from the futuristic world that Timmy Turner has wished for in Future Lost.
Big Brain considers robots to be the superior species and wants to enslave all Humans to serve robots' will. It is housed in a facility in Antarctica and lives in a glass jar which is fed "brain nutrients" that it must suck in to live. This miserable existence drove it to turn on the humans and make being all the robots his hypnotized slaves.
It is the form of a huge human brain floating in a giant jar. The glass jar is connected to two smaller cylinders that have straws leading into the brain jar. Mr. Crocker opens one of these smaller containers, and Timmy mixes it in flavored drinks and ice to create a slush drink, which freezes over the big brain's jar.
The Big Brain lives in a facility in Antarctica. He was created as a result of Timmy Turner's wish for everything in the world to be like what it was in a Stevie Sparks science fiction book. Timmy did not read the book all the way through, and in the plot of the book, an evil overlord called the Big Brain appears and takes control of all robots, turning them against humanity. Timmy enlists the help of Mark Chang and Mr. Crocker to defeat the big brain by turning the giant brain jar that contains it into a big slush drink, giving it brain freeze. The Big Brain's jar freezes over and the control it had over the robots is released, who declare Timmy the Savior of the Future.
If it was not destroyed by that event, it was most certainly unwished when Timmy wished everything back to normal.