also sand-bag, 1860, "furnish (a riverbank, etc.), with sandbags," from sandbag (n.).
The meaning "pretend weakness" is by 1970s perhaps extended from the poker-playing sense of "refrain from raising at the first opportunity in hopes of raising more steeply later" (1940), which perhaps is from sandbagger in the sense of "bully or ruffian who uses a 'sandbag' (in the sense of a cosh or sap) as a weapon to knock his intended victim unconscious" (1882). Hence "to fell or stun with a blow from a sandbag" (1887). Related: Sandbagged; sandbagging.