Definition
This idiom is used to ask what someone is doing or secretly planning, often with a tone of suspicion or disbelief.
I saw him lurking around the office suspiciously, so I asked him, 'What's your game?'
When she noticed a strange transaction on his bank statement, she asked him, 'What's your game?'
They suddenly started asking me all kinds of strange questions, so I couldn't help but ask them, 'What's your game?'