However, no tickets have ever been found which had P.O.S.H on the ticket.
I suggest that the mistake has been made from the full name of the Cruise Liner: Peninsular and Oriental Steam Navigation Company. Maybe a printing error produced P.O.S.H rather than P.O.S.N.C
A more factual defintion still derives from a shipping term which does not seem to be suggested on the previous source sites.
When boats leave a harbour the shipping lanes work to the rule that you must stay to the left hand side like on U.K roads. When leaving a harbour you therefore keep the Port lights of the shipping lane on the left and the Starboard on the right. However, when returning you keep the starboard lights on the left and the Port lights on the right. Hence Port Out Starboard Home... P.O.S.H