There’s no delicate way to put this: if someone says he is going to hit the head, it means he plans on using the restroom. The expression comes from the early sailing navy. Sailors who needed to relieve themselves would make their way to a designated area under the deck near the bow or front of the ship. This area was selected for several reasons. First of all, the odors would be dissipated into the air before reaching the main living and work areas. Secondly, the constant spray of ocean water would act as a natural sanitizer and keep the area relatively clean. Since this area was also close to the carved figurehead on the bow, it became known informally as the head. rochestermilitary.com