To pretend to be weaker than you actually are, especially to gain an advantage in competition; or to hit someone with a heavy object.
From the literal object 'sandbag'—a bag filled with sand used for defense (from the Boer War era and later). The verb developed in the early 1900s with the metaphorical meaning of deception or the literal meaning of striking with a sandbag.
Chess and gaming communities use 'sandbag' constantly—it describes deliberately playing poorly to achieve a lower ranking, then crushing opponents with unexpected strength. It's considered unsportsmanlike, but it reveals how gamers see themselves: the goal isn't fair competition but winning, and if hiding your power helps you win, some see that as clever strategy rather than cheating.
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