One-armed bandit


explanation :
Visitors to Las Vegas should know this idiom.  It refers to casino alot machines: the lever used to play the game is the 'one arm'.  A 'bandit' is a thief, so the idiom refers to idea that many people lose money to the slot machines.  Since Vegas is in what used to be the Old West, it is an especially appropriate idiom.  

e.g. Last night I went to the Mirage and lost a hundred bucks to the one-armed bandits.