The term "shell game" refers to a type of swindling or confidence trick where one person (the conjurer) displays a shell, usually a walnut or hazelnut shell, and shuffles three shells on a table in a way that the observer cannot see which one is real. The mark (or victim) is then invited to bet on one of the shells; if they choose the one containing the nut, they win, but if they choose wrongly, they lose their money or other stakes.
The phrase "shell game" has come to be used more broadly as a metaphor for any kind of deceptive or fraudulent activity where the goal is to cheat someone out of something of value, such as money, time, or trust.