what is a self-replicating rapid prototyper or Rep-Rap?

Answer 1

An idea for a nanobot or robot that would be bad news for the world, depending what it could use as building material.

We really don't want machines replicating in a geometric progression... which they probably couldn't anyway.

In Sci-Fi they seed places with these nanobots and when activated they build combat equipment after making enough of themselves. That's just fiction though and lets hope it stays that way.