poshposh

The story

Most likely derived from Romani 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’) came to refer to money generally. 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 James Payn's The 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." Evidence exists for a slang sense from the 1890s meaning dandy, which is quite possibly related. A popular folk etymology holds that the term is 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.1folk etymology The source marks this as unproven or a folk etymology. Quoted, not asserted. However, there is no evidence for this claim.2folk etymology The source marks this as unproven or a folk etymology. Quoted, not asserted.

Full etymology on Wiktionary