In the military - esp. those services with naval history - Heads refer to where the toilets are located - on sea, or on land. In similar manner, a wall may be referred to as a bulkhead - though the two terms are
technically unrelated.
The term Heads came from the days of sailing ships. It was first used in a nautical sense in
Anglo-Saxon times, where it referred specifically to a ship's figurehead - an ornately carved wooden
decoration located at the front of the ship. Often it was painted in great detail. However, by the 15th century, the term “head” or “boat head” referred to the entire front/bow of a ship, boat, or other vessel.
In time, the term also came to be known by the crew as a place to relieve themselves, which is probably around the same time that they began calling the front of a vessel, the Bow! Unless there was a stinking bucket under-decks, the only place for crew to relieve themselves was at the heads -
all the way forward, squatting on either side of the bowsprit. (The bowsprit being the part of the hull where the carved figure"head" was attached.)