From 《三国志·魏书》: "司马昭之心，路人皆知" — "Sima Zhao's intent [to usurp the throne] is known even to passers-by". The full saying is still quoted to mean "an open secret about someone's hostile agenda".
usage
Often carries an edge of "they're not even hiding it anymore" — more pointed than 人所共知.
The original Three Kingdoms quote this idiom comes from. Sima Zhao was the Wei regent whose plan to seize the throne was so obvious even commoners saw it. Frequently reused in modern commentary about thinly veiled power grabs.
