From How to Shoot Yourself in the Foot in Any Programming Language:
Ruby. Your foot is ready to be shot in roughly five minutes, but you just can’t find anywhere to shoot it.
I don't have experience with Ruby. I guess the first part states that development is fast. What's the meaning of "just can't find where to shoot it" part?
Nice joke! I guess it might refer to the fact that there are very little chances of you going wrong.
It's an old joke. It's not true anymore.
When it was writen, Ruby, and Rails, as you say, have very fast development. The second part refers to the lack of support that Ruby, and subsequently Rails, had across many OSes and Web hosting companies.
As I said, this is just not true anymore.