Silver Bullet (シルバーブレット, Shirubā Buretto) is a name given to Shinichi Kudō and Shūichi Akai by Sharon Vineyard (Vermouth). The name Silver Bullet is probably been given to them as the one who can stop the organization. After Akai's death, she stated that there is only one Silver Bullet who can stop the entire organization which she thinks about Shinichi which has been sized into a small kid after being poisoned by the APTX 4869 and took the pseudonym Conan Edogawa. It is currently unknown what the exact meaning is of the "nickname" Sharon revers them.
Silver Bullet (APTX 4869)
See also: APTX 4869
During the Mystery Train Event, Haibara was listening to the end of the recorded message that was left by her mother in one of her flashbacks. In the recorded message, it was stated that the organization had been working on "a truly terrifying drug" which was something incredible enough which Elena said that they were "putting their hopes on it". That truly terrifying drug was known as the "Silver Bullet". It is unknown what the drug actually is, but it seems to be in connection with the APTX 4869. Currently, Haibara is the only one who knows the secret of the drug which she stated that the drug was something that should have never been created. It can be that the reason why Vermouth actually gave Conan the nickname "Silver Bullet", is probably a reference to the drug. It may also be referenced to the weakness of a werewolf, a beast that is bloodthirsty, similar to the organization. Therefore Shinichi Kudō is their one weakness.