Definition
This idiom is used to ask what someone is doing or secretly planning, often with a tone of suspicion or disbelief.
你在搞什么鬼？
I saw him lurking around the office suspiciously, so I asked him, 'What's your game?'
我看到他在办公室周围鬼鬼祟祟地徘徊，所以我问他：『你在搞什么鬼？』
When she noticed a strange transaction on his bank statement, she asked him, 'What's your game?'
当她注意到他的银行对账单上有一笔奇怪的交易时，她问他：『你在搞什么鬼？』
They suddenly started asking me all kinds of strange questions, so I couldn't help but ask them, 'What's your game?'
他们突然开始问我各种奇怪的问题，所以我忍不住问他们：『你们在搞什么鬼？』