This colloquialism for 'grand', 'swell' or 'first-rate' is said to have originated
in the old days of constant steamship travel between England & India.
Passengers travelling by the P&O (Peninsular & Oriental) would, at some cost,
book their return passage with the arrangement 'Port Out, Starboard Homeward', Thus
ensuring cabins on the cooler side of the ship, as it was usually unbearably hot when
crossing the Indian Ocean.
Passengers would book 'POSH' accordingly, and 'POSH' soon came to be applied to a
first-class passenger who could afford this luxury.