People in the mean alleys and smoky dives of the Brooklyn waterfront know that a sock or small bag filled with sand makes a fearsome weapon, all the better because it leaves no marks. Thus the verb "to sandbag," which since around 1887 has meant "to fell with a blow from a sandbag," or just generally "to bully or intimidate."Source: The Word Detective by Evan MorrisThe specific "hang back" or "slack off" sense of "sandbag" you're wondering about comes from poker, where it originally described a player who held off raising the stakes in order to lull the other players into a false sense of security. The poker sandbagger would pounce late in the game, clobbering the other players with his good hand. More generally, "sandbag" has come to mean to under perform any task in order to gain some advantage.