What POSH means?

The phrase "Port Out, Starboard Home" was originally used to refer to the luxurious cabins on ships that were located on the port side when the ship sailed out of a port, and on the starboard side when the ship returned home. These cabins were considered most desirable because they received the best lighting and ventilation, as well as the most favorable view, because they were on the side of the ship that was away from the docks and other ships in port. 

The term POSH was used as a code by travel agents in booking these cabins, which were typically more expensive than the other cabins.

This phrase became associated with luxury and exclusivity, as the cabins were often booked by wealthy and influential passengers. In the 19th and early 20th centuries, ocean travel was a much more formal and luxurious experience, and these cabins represented the height of elegance and prestige.