- To use the bathroom.
e.g. I'm going to hit the head.
In this case "Head" is the term for the bathroom on a sailing ship.
The Free Dictionary says:
21. (Nautical Terms) nautical
a. the front part of a ship or boat
b. (in sailing ships) the upper corner or edge of a sail
c. the top of any spar or derrick
d. any vertical timber cut to shape
e. (often plural) a slang word for lavatory
Other reference: What Does it Mean When Someone Says They are Going to "Hit the Head"?
I hope that helps.