I think most people here believe in the "Port Out Starboard Home" story. Even if it's not true it sounds true!

To descibe "posh" you have to understand our British obsession with irony. I think that "elegant" might be true but doesn't capture the word. I suspect that "fancy" is a better description as it has more of a dual nature. It's quite a childish word. It has an inverted snobbery overtone. If you said someone was "posh" you'd be more likely to mean pretentious than elegant. On the other hand someone might be quite pleased to be going to a smart occasion and might go out to buy a "posh frock". Translating it to a "fancy dress" rather than "elegant gown" would capture more of the flavour