Brewer's: Secret de Polichinelle
(Le). No secret at all. A secret known to all the world; old news. We have also “Hawker's News,” “Piper's News.” The secrets of Polichinelle are “stage whispers” told to all the audience.
Entre nous, c'est qu'on appelle Le secret de polichinelle.
La Mascotte ii. 12.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894