* Stiffening of the body after death: After death, muscles relax and then stiffen due to a chemical process called rigor mortis. This process makes the body rigid and inflexible, feeling similar to a wooden board.
* Historical usage: The phrase likely arose in the context of handling deceased individuals, whether during burial rites or medical examinations.
* Figurative meaning: Over time, the phrase evolved to describe anything that is extremely rigid, inflexible, or unyielding, beyond the literal context of death.
There's no specific documented origin, but the connection to death and the phenomenon of rigor mortis is quite clear.