• Etymology Today from M-W: thimblerig \THIM-bul-rig\
*1 : to cheat by trickery 2 : to swindle by thimblerig

The game of thimblerig seems innocent enough. The thimblerigger places a seed under one of three thimbles. He deftly scoots the thimbles around on a table, then asks the player to bet on which one hides the seed. (Perhaps the poor bettor is unaware that "rig" has meant "trick" or "swindle" since the 1700s.) But thimbleriggers are masters of sleight of hand and can move and manipulate the seed unfairly -- so the guileless player doesn't stand a chance of winning. When the same sham is played with nutshells, it's called a "shell game."
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