1. conceal or misrepresent something in order to gain an advantage
sandbag your opponents/rivals
This expression is mostly used in a professional context and is a strategy with the aim of lowering the expectations around something or someone in order to produce better-than-expected results. To do so, it is not always necessary to lie, you can instead withhold information, or be as conservative with your estimate as possible. In other words, only provide the worst-case scenario. In a social respect, it tends to be used in relation to betting or sport. As a verb to 'sandbag' someone is to deceive someone in order to get what you want or deliberately underperform in order to gain an advantage.