Etymology

Unknown; popularly believed to be an acronym for "port out, starboard home", describing the cabins given to upper-class passengers travelling with the Peninsular and Oriental Steam Navigation Company from Britain to India and back. The company denies this practice, despite christening its loyalty scheme the "P.O.S.H. Club" after the myth (the club has since been renamed "Portunus").

A more likely origin is the Romany word posh meaning "half-", as in posh-kooroona meaning "half a crown" - a once-substantial sum of money, and hence by association anything pricey or upper-class. Alternatively posh may have first become a general term for money, after posh-houri, half-penny.
See World Wide Words article on "posh" for other theories.

A period slang dictionary defines "POSH" as a term used by thieves for "Money : generic, but specifically, a halfpenny or other small coin." An example is given from Page's Eavesdropper, 1888: "They used such funny terms: 'brads,' and 'dibbs,' and 'mopusses,' and 'POSH' ... at last it was borne in upon me that they were talking about money." It is noteworthy that POSH is spelled in capital letters, like the alleged acronym.

Adjective

1. Associated with the upper classes.

   She talks with a posh accent.

2. Stylish, elegant, exclusive (expensive).

   After the performance they went out to a very posh restaurant.

3. Snobbish, materialistic, prejudiced, under the illusion that they are better than everyone else. usually offensive. (especially in Scotland)

   We have a right posh fart moving in next door

Quotations

• 1919: "Well, it ain't one of the classic events. It were run over there." Docker jerked a thumb vaguely in the direction of France. "At a 'Concours Hippique,' which is posh for 'Race Meeting.' — Punch, or the London Charivari, Vol. 156, June 18, 1919

Quotations

• 1889: "The czar! Posh! I slap my fingers--I snap my fingers at him." — Rudyard Kipling, The Man Who Was