The term sandbagging generally refers to a cheat who conceals his real talents to fool his opponent. In cards, a sandbagger pretends his hand is lousy to get other players to put greater stakes on the table. In golf, a sandbagger pretends he has a lower level of ability than he really does. In pool, think Paul Newman in "The Hustler": a pool shark acting like an amateur to raise the bets against him. Though the origin of the word sandbagging didn't mean quite the same thing, it did have a negative definition that has carried on through the years.
Back in the Day
The word ''sandbag'' dates back as far as the 1580s, according to the Online Etymology Dictionary, and referred merely to a bag filled with sand, often used as a weapon. The noun evolved into a verb around 1887 meaning to attack someone with a sandbag, making a sandbagger a rotten individual who ambushed his victims with that sandbag. In the 1940s, sandbagging meant to hold back from betting at first in hopes of making a greater bet later. The modern meaning of the term took shape around 1970.