I believe the word posh comems from the days of transatlantic ship travel when rich people could afford to travel “Port Out Starboard Home” to avoid being in the glare of the sun both ways on their voyage. If you had less money you could not afford to pay the POSH premium and so were not considered “posh”. Someone please correct me if I’m wrong.

Thats the general idea, although I think it is supposed to be related more to shipping going to the Far East and Australia.

I have a feeling that it is another one of these urban myths though.