Posh

Naval This word, meaning 'superior', is said to come from the P. & O. Steam Navigation Company's abbreviation for the phrase 'Port Outward, Starboard Homeward', which cabins are the cooler in the Red Sea and so the more attractive to passengers.
Found on http://www.britishempire.co.uk/glossary/p.htm

posh

nonsense
Found on http://phrontistery.info/p.html

Posh

Posh is British slang for stylish, elegant,
Found on http://www.probertencyclopaedia.com/browse/ZP.HTM

Posh

Roughly translates as high class, though if you look at Posh Spice there are clearly exceptions to the rule! Comes from the cabins used by the upper class on early voyages from England to India. The coolest (and most expensive cabins) were Port side on the way Out and Starboard on the way Home.
Found on http://www.effingpot.com/slang.shtml
No exact match found