From Longman Dictionary of Contemporary Englishsecret weaponsecret weaponsomething that will help you gain a big advantage over your competitors, that they do not know about → secret
Examples from the Corpus
secret weapon• And Bannister, who weighs 22 stone and has size 17 feet, could be Cadle's secret weapon.• Further, there was the frightening possibility of new secret weapons.• So long as she stayed silent she had a secret weapon.• Yet Drake had been aided by a secret weapon.• His secret weapon has been a three-wood he first used last June.• But his real secret weapon is an amazing talent for simultaneously combining slide with fretted notes.• The next day she carried her secret weapon to school in her satchel. 