Trump said this when referring to Jeb Bush. He invoked his former Republican rival because “he’s open game” for not standing by his pledge to support the GOP nominee. “So he signed the pledge, right? But he didn’t honor it,” Trump said. “How do you — how do you live with yourself, you know?” http://www.politico.com/story/2016/10/donald-trump-jeb-bush-230358
This is a mash up of “open season” (a period of time when everyone is criticizing someone) and “fair game” (someone that is considered permissable to attack or abuse). Both idioms involve criticizing someone or something which is the cause of the mix up. A big thanks to Frank King for hearing this one and sending it on!