More evidence of my nerdery. I just spent half an hour searching Google and I've found out the name of the tree (osage orange or Maclura pomifera) and a picture of the fruit.
I never found a specific name for the fruit, so I will continue to call it a monkey brain.
To read more about the osage orange, and its origins, go here. (I'm sure you will)
Thus ends today's science lesson.