No silver bullet
A "silver bullet", as legend has it, is the only thing that can kill a Werewolf. Generally speaking, a silver bullet is a simple but guaranteed solution for a difficult problem.
In his famous essay of 1987, Silver Bullet - Essence and Accidents of Software Engineering, Frederick P. Brooks talked about the silver bullet of software development. Brooks argues that there will be no individual technology or practice, that would make a 2-fold improvement in productivity within 2 years. So the werewolf he had in mind is the software productivity monster, which many programming languages, environments, and methodologies had promised, but ultimately failed to slay.
For some people "no silver bullet" means that software engineering in general failed. The search for a single key, a so called "wonderweapon" (direct German translation of silver bullet), to success never worked. All known technologies and practices have only made incremental improvements to productivity and quality. Although there are no silver bullets for any other profession either. Others interpret "no silver bullet" that software engineering has finally matured and recognized that projects succeed due to hard work.