Posh

According to Yahoo Education

'posh' means smart and fashionable. a popular theory holds that it is derived from initials of 'port out, starboard home'. the cooler and expensive side of the ships travelling England and India in mid 19th century. posh was supposedly stamp on the tickets of first-class passengers travelling in that side of ships owned by the peninsular and oriental steam navigation company.

Reference 

Education.yahoo.com, 'posh' Available at:: http://education.yahoo.com/reference/dictionary/entry/posh (Accessed : 8 December 2011).

According to oxford dictionary 

Early 20th century: there is no evidence support like the flock etymology that posh is formed from the initials of port out starboard home referencing more confortable accommodation out of the heat of the sun, on ships between England and India.

Reference

Oxford dictionaries.com, (2011)'posh' Available at : http://oxforddictionaries.com/definition/posh (Accessed : 8 December 2011).