noun, masculine/sə.kʁɛ d(ə) pɔ.li.ʃi.nɛl/plural: secrets de Polichinelle
secret de Polichinelle is masculine in French.
open secret, secret of Polichinelle
“A six mois du scrutin municipal demeurent quelques secrets de Polichinelle dans la capitale comtadine.”
Literally, “secret of Polichinelle”, from a play of the same name by Pierre Wolff (1903), about a bourgeois family trying to conceal the “open secret” that their married son has a child born out of wedlock.