(as) stiff as a board

Very straight, rigid, or inflexible. Used to describe someone's physical posture or demeanor. The whole class sat in their chairs as stiff as boards during the principal's lecture. He stood as stiff as a board after his name was called, too nervous to move. Remember to stand as stiff as a board during the ceremony. They don't want to see any cadets slouching.

Also, stiff as a poker. Inflexible, rigidly formal, unbending, as in This cloth is stiff as a board; what happened to it? or There he stood, stiff as a poker, unwilling to give an inch. The board in the first simile for rigidity is a slab of wood; the second, alluding to the iron implement used to push around logs in open fires, dates from the late 1700s.

If you or your body are as stiff as a board, you are very stiff. You'll achieve flexibility very quickly with these exercises — even if you're as stiff as a board at your first session. His lower back felt as stiff as a board.

Rigid, inflexible. This common simile for being unbending is replacing the earlier stiff as a poker, probably because central heating has made fireplace implements like pokers less common household items. Stiff as a poker dates from the eighteenth century; it appeared in numerous sources, such as “Stuck up as stiff as a poker” (George Colman, Jr., The Heir at Law, 1797). Earlier still was stiff as a stake (sixteenth century), which is now obsolete.