Batten down the hatches

Actually, it's a skylight, and it's ajar. We don't have any hatches.

The hatch referred to was a hatchway, or opening, in a ship's deck. They were usually open or covered with a wooden grating, so bad weather would cause problems.

The batten is a lath, or strip of wood. They were used to weigh down the edges of a tarpaulin placed over the hatch when bad weather approached.

Hence, to batten down the hatches. Nowadays, its meaning is figurative: to prepare for trouble.

Technically, we don't actually have any hatches. This doesn't stop us calling the slidy thing above the stern doors a hatch.