The Macmillan English Dictionary for Advanced Learners defines the word posh as follows:

  1. something that is posh looks expensive and attractive.
  2. someone who is posh talks and behaves in a way that is typical of people from a high social class.

It is widely believed that this word has its origins in the days of the British Empire when members of the civil service and armed forces would travel out to India to do their duty, often taking family members with them. The journey by sea was quite arduous and in order to make it more pleasant, wealthier passengers would buy the more shaded cabins on the port (left) side of the ship on the voyage out to India. This kept them out of the sun. Likewise, on the return journey they would buy cabins on the starboard (right) side of the ship. Thus the story went round that posh is an acronym for ‘port out, starboard home’. It is a nice theory but unfortunately one that has never been proved. In fact the Oxford Concise Dictionary of English Etymology states simply that the word posh is “of unknown origin”. 

The word posh only functions as an adjective: in very colloquial British English, a posh person is sometimes referred to as a posho.