These are some saying reputed to come from sailing. Because the origins of saying can be obscure, their origin may be at times disputed.
Loose cannon: If a cannon was being fired on a ship and it broke free from its securing ropes, the recoil would pose a danger to both the ship and crew.
Today: Used to describe someone dangerous and unpredictable.
At a rate of knots: To measure speed at sea, knots were tied on a piece of rope at regular intervals, which was dropped overboard. As the rope unwound from a reel, the number knots that passed calculated the speed of the vessel.
Today: To do something very quickly.
Take the wind out of their sails: A ship moves in front of another ship and blocks the wind, it could slow right down or stop completely.
Today: To discourage or deflate someone.
Groggy: Grog was the daily ration of rum for sailors. When sailors had too much to drink, they would be called groggy.
Today: When someone is sleepy or disorientated.
Plain sailing: Sailing is proving easy and successful.
Today: Everything is going well.
A leading light: When docking at night, lights on shore helped guide ships safely into port, and a leading light was placed prominently to assist.
Today: Someone well respected in a particular field.
As the crow flies: It is suggested Vikings released crows to help find a route to the nearest shore.
Today: The direct distance between points A and B.
On an even keel: The keel is the primary structural backbone of a boat, running lengthwise along the bottom of the hull. If the keel is even then the ship is stable in the water.
Today: Describe something as steady and predictable.
Batten down the hatches: When a storm was on its way, the captain would give this order.
Today: Prepare for something negative about to happen.
Not enough room to swing a cat: There was a whip called the cat-o'-nine-tails. This was a painful naval punishment. It was usually too cramped to swing the whip below deck.
Today: Describes a space too small to do what you want.
Cut and run: A ship may cut the rope connected to the submerged anchor to get away suddenly.
Today: Cut your losses and get out of a situation or leave very suddenly, even if it cost you to do so.
Three square meals a day: A wooden plate was used by sailors to eat their food from.
Today: Denotes getting three good, adequate meals a day.
On the fiddle: The above plates had a raised edge called a fiddle, to contain the food. The fiddle (the raised rim) kept the food on the plate. If too much food was on a plate and was touching or overlapping the fiddle, the individual had taken too much food. A serious offence in the Navy.
Today: Someone doing something dishonest.
