The expression hit the head 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 hit the head 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. http://www.wisegeek.com/what-does-it-mean-when-someone-says-they-are-going-to-hit-the-head.htm

The "head" is the nautical slang for the bathroom on a ship. I can only assume it's because it was at the head of the ship, meaning the front. Although not much nautical jargon makes a lot of sense, so I could be wrong about that part.

The most popular phrase is - "Hit the nail on the head": Meaning: Get to the precise point. Do or say something exactly right. Origin: No one knows the exact origin of this phrase. What is known is that it is extremely old. It appears in The Book of Margery Kempe, circa 1438. This was an account of the life of religious visionary Margery Kempe and is considered to be the earliest surviving autobiography written in English: "Yyf I here any mor thes materys rehersyd, I xal so smytyn ye nayl on ye hed that it schal schamyn alle hyr mayntenowrys." In modernised English, that reads as: "If I hear any more these matters repeated, I shall so smite the nail on the head that it shall shame all her supporters." Kempe's meaning in that citation isn't entirely clear. Some have interpreted her 'hit the nail on the head' as 'speak severely'. The current 'get to the heart of the matter' meaning is unambiguous in a later reference, from the end of the following century. Henry Buttes' Dyets drie dinner, 1599, includes this line: "His chiefe pride resteth in hitting the nayle on the head with a quainte Epithite." http://www.phrases.org.uk/meanings/hit-the-nail-on-the-head.html to describe exactly what is causing a situation or problem. to be right about something. http://idioms.thefreedictionary.com/hit+the+nail+on+the+head Also, hit the mark or the nail on the head. Be absolutely right, as in Your remark about finances hit the bull's-eye, or Jane hit the mark with her idea for shuffling personnel, or The governor's speech on attracting new businesses hit the nail on the head. The round black center of a target has been called a bull's-eye since the 17th century; mark similarly alludes to a target; and the analogy to driving home a nail by hitting it on its head dates from the 16th century. http://www.answers.com/Hit%20the%20nail%20on%20the%20head Idiom: off the mark Also, wide of the mark. Inaccurate, wrong, as in The forecast was off the mark, since unemployment is down, or His answers on the test were just wide of the mark. It is also put as miss the mark, meaning "be mistaken," as in The minister missed the mark when he assumed everyone would contribute to the supper. All these terms allude to mark in the sense of "a target," as do the antonyms on the mark and hit the mark, meaning "exactly right," as in He was right on the mark with that budget amendment, or Bill hit the mark when he accused Tom of lying. [Mid-1300s] http://www.answers.com/topic/off-the-mark (Slang) hit the head Going to the bathroom for uruinals. For a girl to give a man oral/head. http://www.urbandictionary.com/define.php?term=+hit+the+head There's no delicate way to put this – if someone says they are going to hit the head, it means they plan on using the restroom. The expression hit the head 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 hit the head 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 hit the head 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 our 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. If a situation is considered satisfactory or pleasant, it is often said to be hunky-dory. 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. So if you're three sheets to the wind, or need to hit the head to get the latest scuttlebutt, watch out for the crow's nest. I hear it packs a mean wallop. http://www.wisegeek.com/what-does-it-mean-when-someone-says-they-are-going-to-hit-the-head.htm "Hit the head" is more popular as a slang than a traditional phrase.