silver bullet

noun4

1. •
   A bullet made of silver, usually with reference to the folkloric belief that such bullets are the only weapons which can kill a werewolf, vampire, or other monster.
2. •
   (figurative) Any straightforward solution perceived to have great effectiveness or to bring miraculous results.

   • There is no silver bullet for this problem. It will take hard work and long hours to fix.
3. •
   (informal) A classic dry martini cocktail.

   • The Martini is nicknamed “the silver bullet” because of its supposedly lethal properties. — 2003
4. •
   (slang) A thermometer, especially one used for rectal temperature readings.

   • […] "the silver bullet"—a rectal thermometer, to ensure that their body core isn't dangerously overheated from the run. — 1998