The term POST ON A SHIP can be broken down into two words: MAST (4 letters) and POST. The MAST of a ship is the main vertical pole that supports the sails and rigging. It is usually the tallest structure on the ship and is used to hoist flags and signal other vessels. Posting on a ship is the act of attaching a notice or announcement to the MAST, usually for the purpose of informing the crew or passengers of important information.