Green’s Dictionary of Slang

1. a street robber, a ‘mugger’.

1882		G.W. Peck Peck’s Sunshine 203: Suppose all the men that have been robbed in the past year by cowardly sandbaggers, could have ‘put up their hands’.
1884		G.W. Peck Peck’s Boss Book 72: I am going to pull sand-baggers and horse-thieves for eighty dollars a month.
1898		Binstead & Wells Pink ’Un and Pelican 254: The very lowest characters of both sexes — bunco-steerers, gold-brick fabricators, sandbaggers, and, worse than all, if that be possible, the alien dagos from Italy and Spain.
1901		C.R. Wooldridge Hands Up! 66: Confidence men, shell workers, and sand-baggers followed them like wolves after a lone prairie traveler.
1906		A.H. Lewis Confessions of a Detective 33: Where the sandbagger crept upon me was midway between two lamps, and a darkest spot along the street.
1915		Wodehouse Psmith Journalist (1993) 248: Would it be betraying professional secrets if you told us which particular bevy of energetic sandbaggers it is to which you are attached?
1929		C.E. Merriam Chicago 343: A matter to be carefully watched here is room for blackmail, even in the case of worthy measures unless the sandbaggers are offset by those of an opposite persuasion [DA].

2. one who is believed to win unfairly in a game or sport involving betting.

1961		(con. 1945) G. Forbes Goodbye to Some (1963) 91: Prime fans his cards on the table. He has aces and queens. ‘You, you goddam sandbagger,’ the major says bitterly.
2018		M. Mesko Confessions of a Caddie 46: Sandbagger[:] Used to describe a golfer who artificially increases their handicap; a cheater.