lit. the assassin's dagger, concealed in a map scroll, is suddenly revealed when the map is unrolled (referring to the attempted assassination of Ying Zheng 嬴政[Ying2 Zheng4] by Jing Ke 荊軻|荆轲[Jing1 Ke1] in 227 BC) (idiom) / fig. malicious intent suddenly becomes apparent