first rule of (something): do not talk about (something)

cliché A phrase used to highlight the need to keep some group or piece of information a secret. Often used humorously or sarcastically. Modeled on the quote from the book Fight Club and its film adaptation, "The first rule of Fight Club is you do not talk about Fight Club." Right, welcome to Coding Club, new recruits. The first rule of Coding Club: do not talk about Coding Club. Haha, just kidding—a little joke for the film buffs here. We made a pact to take the information with us to our graves. First rule of the pact: do not talk about the pact. First rule of poker night: do not talk about poker night, especially to any wives or girlfriends, got it?