Similar to what happens in Vegas stays in Vegas. If you are having a private conversation that is not to be repeated you say "Vegas Rule".

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"Vegas rules" is a phrase that comes from the a story where a gambler in Vegas cannot pay his debts and has his arm broken for his trouble. it's Common meaning simply states that if you don't complete a certain task ( Normally paying your share of a bill) "Vegas rules" means an unpleasant and usually violent thing happens to the offending party