The word 甲板 is formed by combining two characters: 甲, which originally means a hard shell or armor (like that of a turtle or in ancient battle gear), evoking the idea of a protective, outer covering; and 板, which means a plank or flat board. Together, they create a conceptual compound where the "armor-shell" layer (甲) made of "planks" (板) metaphorically describes the strong, planked, outer walking surface of a ship, the deck.