trump card
noun [ C usually singular ] uk/ˈtrʌmp ˌkɑːd/ us/ˈtrʌmp ˌkɑːrd/
独有的优势，独门法宝；王牌；绝招
an advantage that makes you more likely to succeed than other people, especially something that other people do not know about
Anthea was about to play her trump card - without her signature none of the money could be released. 安西娅准备打出她的王牌——没有她的签字，一分钱也不能动。