‘Knot’ goes right back to the mid-1500s when sailors would cleverly tie knots in a piece of rope at regular intervals. They would then throw one end of the rope, which was weighted, into the sea behind their moving boat. The other end was kept onboard the vessel, probably wound up in a reel of some sort. The sailors would then count the number of knots that were let out in a given period, probably measured by an hourglass. They would then use this finding as a calculation of speed which they could also use to approximate how long it would take them to get somewhere in the given conditions.
So the story of the word ‘knot’ goes right back to clever sailors and it’s been used ever since, even when technology came and made calculating speed at sea a bit simpler.