Hit the Deck is an idiom that means to take immediate action. It is often used as a warning to get out of the way or to take cover. The crossword solutions for this term are SWAB, which is a short form of swabbing the deck, which is a nautical term meaning to clean the deck of a ship, and FALL, which is a reference to the phrase 'FALL flat on your face' which is another way of saying to take immediate action.