(as) stiff as a poker
Very straight, rigid, or inflexible. Used to describe someone's physical posture or demeanor. The whole class sat in their chairs as stiff as pokers during the principal's lecture. He stood as stiff as a poker after his name was called, too nervous to move.
*stiff as a poker
rigid and inflexible; stiff and awkward. (Usually used to describe people. *Also: as ~.) This guy's dead. He's cold and as stiff as a poker. John is not a very good dancer; he's stiff as a poker.
(as) stiff as a ˈpoker
(informal) (usually of people) very straight or upright in the way you sit or stand: The old lady was sitting upright in her chair, stiff as a poker.