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 navy and coast guard jargon for bathroom. Sailors, marines and Coast Guard members call their facilities heads, while land-based military personnel call them latrines. Naval ships actually have the word head stenciled on the watertight doors leading to the cramped but serviceable facilities.
The origin of this phrase can be traced back to ancient sailing vessels. 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. The term stuck even as shipbuilders incorporated indoor plumbing and other modern conveniences to military ships. Generations of sailors have since adopted the phrase hit the head as a euphemism, and eventually the term became part of popular culture as these men and women assimilated back into society.
The phrase is just one example of military jargon entering popular usage. There are a number of other naval terms and expressions that may sound very familiar to land-bound ears, such as the word wallop. It is said that King Henry VIII sent an Admiral Wallop to France in order to avenge the French burning of the city of Brighton. The resulting damage to the French coast was so severe that Wallop's name became synonymous with the use of overwhelming force.
Other familiar phrases with a naval origin include hunky-dory, supposedly a corruption of Honki-Dori, a Japanese street known for its hospitality towards sailors on leave. A slang word for office gossip, scuttlebutt, is also said to come from naval history. Sailors stored their drinking water in stoppered barrels called scuttlebutts, or simply butts. Time spent around these water barrels would often involve the retelling of rumors or other bits of ship's news.