poker-faced

Etymology
From the attitude necessary in the game of poker, since one is ill-advised to reveal delight at being dealt a winning hand lest other players decline to bet (or contrariwise, show disappointment at a poor hand if one is inclined to bluff).

Adjective

 * 1) Wearing a poker face: studiously neutral in facial expression; giving nothing away.
 * John, in his most poker-faced manner, continued to explain the rules.