secret weapon
noun [ C ] uk/ˌsiː.krət ˈwep.ən/ us/ˌsiː.krət ˈwep.ən/
秘密武器；杀手锏
something or someone that no one knows about and that will give you an advantage over your competitors or enemies
Johann was the bank robbers' secret weapon - he knew how the security system worked. 约翰是银行劫匪的秘密武器——他对警报系统了如指掌。