A ramrod is a long rod used to pack gunpowder and ammunition down the barrel of an old-fashioned gun. Before the invention of modern firearms, soldiers and hunters had to load their muskets and rifles from the front of the barrel. They'd pour gunpowder down, drop in a lead ball, then use the ramrod to push everything firmly into place. Without a ramrod, the gun wouldn't fire properly.

Because the ramrod had to be straight, strong, and reliable, we started using the word to describe people who are extremely strict, rigid, or formal. If someone has ramrod posture, they're sitting or standing perfectly straight, like a soldier at attention. A ramrod boss runs things with inflexible discipline.

You might also hear the phrase ramrod straight to describe something perfectly vertical or someone standing without the slightest slouch. The image is of that essential tool: unbending, reliable, and always ready to do its job exactly the same way every time.