"Batten down" is a 19th-century nautical term meaning to secure a ship’s deck hatches against heavy weather. Sailors used long wooden strips, called battens, to fasten tarpaulins over cargo openings to prevent water from flooding below decks. It often refers to the phrase "batten down the hatches."

Key Aspects of the Term:
Action: Crew members would nail or fasten wooden strips (battens) over canvas coverings on hatches (openings).

Purpose: To make the ship's deck as watertight as possible, stopping waves from flooding the hold and reducing the risk of sinking.

Origin: Originated in the early 19th century as a literal survival technique during storms at sea.

Modern Usage: It now often refers metaphorically to preparing for an imminent emergency or period of difficulty.