Posh - Elegant, swanky, rich.
The much-repeated tale is that 'Posh' derives from the 'port out, starboard home' legend supposedly printed on tickets of passengers on P&O (Peninsula and Orient) passenger vessels that travelled between UK and India in the days of the Raj. Britain and India are both in the northern hemisphere so the port (left-hand side) berths were mostly in the shade when travelling out (easterly) and the starboard ones when coming back. So the best and most expensive berths were POSH, hence the term.
There is no evidence to confirm this story though and it appears to have been dreamed up retrospectively to match an existing meaning.