Noun. Relevence to "Mosh Pit". A gig, or perhaps a party, where the majority of people attending are extremely middle classed and are sucking up to each other and the band way too much. It is usually not an enjoyable experience for someone who is not within the "social circle" of the "Posh Pitters".
Often at a Posh Pit, people attempt to create a mosh pit simply for the sake of it, because its the done thing, not because the music suits it.
A: "I was at a party last night and it sucked. Everyone was from Twickenham and they were dancing like dickheads. They tried to make a mosh pit and it just failed, someone even tried to stage dive onto a group of about 5."
B: "That sounds like one hell of a Posh Pit"