1590, from sand + bag. The verb sense of "pretend weakness" is 1970s, extended from 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 as a weapon to knock his intended victim unconscious" (1882).
To attack someone viciously, esp with a blackjack or similar bludgeon; bushwhack: I was sandbagged from behind(1887+)
To intimidate; cow; bulldoze: Persuasion didn't work, so they tried to sandbag her(1901+)
To check and then to raise the bet (1940+ Gambling)
To pretend weakness or ineptitude; mislead an opponent by apparent inferiority: He charged that the Aussies were ''sand bagging'' (deliberately losing) to take the limelight off their disputed keel(1970s+)
To drive a hot rod very fast (1950s+ Hot rodders)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source