Poker Face

An Americanism from the game of poker, where players cultivate a deliberately expressionless face so as not to give opponents any hint of the strength of the hand they're holding. The term is documented from the late 19th century and had crossed into general figurative use, describing anyone hiding their true feelings behind a blank expression, well before the 20th century. Its most famous modern moment came over a hundred years later, when Lady Gaga's 2008 song of the same name brought the old poker-table phrase back into everyday conversation for an entirely new audience.