司马昭之心

From the 《三国志·魏书》 and later 《汉晋春秋》. Sima Zhao (司马昭, 211–265) was the de facto ruler of Wei at the end of the Three Kingdoms; everyone knew he intended to usurp the throne for his family (which his son Sima Yan did, founding the Jin dynasty). The young puppet-emperor Cao Mao is recorded as saying 司马昭之心，路人所知也 — 'Sima Zhao's heart is known even to passers-by.' The phrase became proverbial for a transparently malicious ambition.

Paired couplet — almost always followed by 路人皆知 ('known to every passer-by'). Used in political, business, and everyday contexts for motives someone thinks they are hiding but clearly aren't. 昭 is zhāo (first tone), a classical word for 'bright' preserved here as a personal name.