POSH stands for "Port Out, Starboard Home" in nautical terminology. Posh is a word of unknown derivation but there is no evidence that it ever stood for the custom of a Steamship line (usually P&O) stamping tickets POSH to ensure a shaded journey to and from India. There are no examples of Acromyms before the 20th Century the word posh is older its possible derivation is from the Romany cant word for money.
Posh means that you come from a well-off established amily, are well-educated, and has snobbish connotations. For example, my father might be considered posh, because he went to Charterhouse public school, then Oxford University, as well as speaking with 'Received Pronunciation'. In the UK, working-class youths often mock those higher in class with the insult 'posh tw*t'.
I heard that the word Posh originated as the description of a cabin on an ocean liner. Port Out Starboard Home. I think a Posh cabin was situated on the side of the ship that gave you morning sun and this would be on the opposite side of the ship for the return or home journey. Posh therefore meant above average, smart, desireable