Because sailboats and [nautical miles](https://en.wikipedia.org/wiki/Nautical_mile).
To measure their speed, sailors would traditionally tie knots in a piece of rope and let it out the back of the boat. They would then count the number of knots that passed through their hand as the rope was pulled backwards relative to their motion.
On boats, to calculate speed, a tool called a chip log would be used. The chip log was a long rope with knots regularly along it (wrapped around a spindle) and a large wood “chip” at the end. The chip would be thrown into the water and however many knots passed through the users hand would determine the speed.
There are various records of how far apart the knots were or how long it was timed for.
Here’s something else to throw into the mix.
First, **knot** is really just “slang” for **naut**ical mile (per hour); and -mile- is one of those words for which there are multiple meanings — like degree or ton/tonne.
You can’t just say — unambiguously — “It’s 35 degrees out”, because that could be Hot in one place (°C), Cold in another place (°F), and Incomprehensively, Out-of-this-World cold on the back side of the new Webb Space Telescope (°K). You have to know which kind of degree.
And you have to know if a ton(ne) is 2000 pounds, or 1000 kilograms, or a long ton, or a metric shitton … you get the picture.
And so it it with miles. To be perfectly clear, you should always use the adjective — **statute** miles, or **nautical** miles. But over time, we came to call the latter “nauts” or “knots” (per hour).
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