The term dates back to at least the 1970s, when it was used among cinematic special effects artists to refer to physical models used in the same way in the construction of props and miniatures. At that time, they were often made from kitbashed hobby models of battleships, tanks, and cars.
Today, the term usually refers to digital 3d models of this nature, which are created in the same manner as any other 3d model, but reused repeatedly to speed up the process of creating subsequent models.
It can also be used as a verb to describe the act of applying nurnies.
I don't want to design any spaceships today, so I say for these Star Destroyers, we nurnie these enormous arrow shaped piece of foamcore. Who's with me?
That robot looks cool, but it doesn't seem technologically complex enough. Nurnie it up, buttercup.