“”I define anarchist society as one where there is no legal possibility for coercive aggression against the person or property of any individual.
|—Murray Rothbard, who kept a case of silver bullets in his closet|
A silver bullet is any proposal or belief that rests on some one simple thing being the root cause of a massive and complex problem. It is a favorite device of the intellectually lazy who do not want to deal with your petty complications and nuance. Silver bullets show up in fake science and fake economics, and should be an immediate red flag that somebody is desperately trying to find rationale for a conclusion they arbitrarily chose at some point in their adolescence.
The term is derived from stories of werewolves and witches, where a silver bullet is required to kill the beast. Since silver bullets are effective for killing just about anything in your average monster story, it is also used to describe an idea which should "fix everything." 
- No Silver Bullet: Essence and Accidents of Software Engineering by Frederick P. Brooks, Jr.