Johnny:

I had to go to the Brits -- the OED Web site in fact -- to find this:

The story goes that the more well-to-do passengers travelling      to and from India used to have POSH written against their bookings, standing      for 'Port Out, Starboard Home' (indicating the more desirable cabins, on the      shady side of the ship). Unfortunately, this story did not make its appearance until the 1930s,       when the term had been in use for some twenty years, and the word does not       appear to have been recorded in the form 'P.O.S.H.', which would be expected       if it had originated as an abbreviation. Despite exhaustive enquiries by       the late Mr George Chowdharay-Best, researcher for the OED, including       interviews with former travellers and inspection of shipping company documents,       no supporting evidence has been found.

Sorry, mate!

A