phrase/lə ʒy.pɔ̃ de.pas/
one's hidden intentions or an embarrassing truth have been inadvertently exposed; one is showing one's true colours.
“2006, Canada. Parliament. House of Commons, House of Commons Debates, October 3, 2005 (Hon. Pierre Pettigrew): [Le Bloc québécois] est absolument incapable de faire un travail constructif et d'améliorer la fédération. Il a prétendu vouloir améliorer le fonctionnement de notre fédération. Son jupon dépasse. [The Bloc Québécois] is completely unable to do constructive work and improve the federation. It has claimed to want to improve how our federation works. Its true colours are showing, however.”
Literally, “one's slip (underskirt) is showing”.