The 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. -The Word Detective
the reference to "clobbering" gets at the ultimate origin, which is the use of a sock filled with sand to strike someone, the object of this being to inflict pain without leaving a mark.
>Aha! I was right! Sandbagging should refer to the act of performing up to capabilities not the act of laying back in preparation.
was anyone else as nonplussed by this sequence as I was? if not, I will consider myself sandbagged and cash in my chips.
