Idiom
be as stiff as a board
1.
to be very stiff.
It's so cold out there - the washing was as stiff as a board when I brought it in off the line.
2.
if you are as stiff as a board, your body feels stiff and hurts when you try to move it, usually after a lot of physical exercise.
I cycled fifty miles yesterday and when I woke this morning I was as stiff as a board.