Popularly believed to be an acronym for "port out, starboard home", describing the cooler, north-facing cabins taken by the most aristocratic or rich passengers travelling from Britain to India and back.
More likely derived from the Romani term posh (“half”), either because posh-kooroona "half a crown" (originally a substantial sum of money) was used metaphorically for anything pricey or upper-class, or because posh-houri "half-penny" became a general term for money.
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."
See this World Wide Words article on "posh" for other theories.
- Associated with the upper classes.
- She talks with a posh accent.
- Stylish, elegant, exclusive (expensive).
- After the performance they went out to a very posh restaurant.
- Snobbish, materialistic, prejudiced, under the illusion that they are better than everyone else. usually offensive. (especially in Scotland and Northern England)
- We have a right posh git moving in next door
- 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
- An exclamation expressing derision.
- 1889: "The czar! Posh! I slap my fingers--I snap my fingers at him." — Rudyard Kipling, The Man Who Was
- ^ http://www.bbc.co.uk/news/magazine-22378819
- ^ Slang and its Analogues Past and Present, volume 5 (London, 1902), John S. Farmer and W.E. Henley (editors), page 261
Alternative forms 
From Armenian փոշի (pʿoši).