The phrase BATTEN DOWN THE HATCHES means to prepare for trouble. It has a nautical background.
‘Hatch’ is an ‘opening in the deck of a ship’ meaning. Ships’ hatches, more formally called hatchways, were commonplace on sailing ships and were normally either open or covered with a wooden grating to allow for ventilation of the lower decks.
When bad weather was imminent, the hatches were covered with tarpaulin and the covering was edged with wooden strips, known as battens, to prevent it from blowing off. Not surprisingly, sailors called this ‘battening down’.