To “batten down the hatches” refers to making preparations in order to withstand approaching trouble. The idiom means that whatever is needed to survive a disaster, whether it be shelter, property, or a bank account, has been made as secure as possible. When someone is told to batten down the hatches, he or she is being told to make necessary precautions for an approaching problem, figuratively, or an approaching storm, literally.
The phrase batten down the hatches entered language from the high seas. When a storm approached or a ship was about to enter rough waters, the hatches were battened down to help the vessel withstand the bad weather and not take on water. Hatches cover openings on ships’ decks, and when they were battened down they were closed, nailed, and caulked shut to prevent water from entering the ship during the storm or while passing through rough seas. The batten was a wooden strip nailed across the hatch.
The hatches were also the coverings for the holes that led to the living quarters of those aboard the ship. This meant battening down the hatches would protect the areas in which those on the ship would take refuge during the storm. A lookout high above the deck in a crow’s nest would be the first to notice the approaching storm or rough waters. This person was likely to alert the rest of the crew of the coming problem with the command, “Batten down the hatches.”
The idiom seems to have been introduced into use in everyday language in the 19th century, though it may have been used much earlier than that by those who made their living on the open seas. The first written reference to the practice of nailing closed and caulking the hatches is found in an 1823 work by John Badcock. Called Domestic Amusements, Badcock's work doesn't use the actual idiom, but describes the process of preparing for the storm. The actual saying doesn’t appear in written records until 60 years later.
The phrase is commonly in use now, though in a figurative sense. It is likely that many who use it, and even many who understand what it means figuratively, don’t know the literal meaning of the word. Forecasters often use the phrase when they warn viewers or listeners of approaching severe weather, especially hurricanes. Those in an area likely to be hit by storms are often told to batten down the hatches before the weather arrives.