The space left in your wallet after a particularly drunken and expensive night out. Normally after being invited on an aquaintances birthday/stag night to go somwhere you would not normally go due to the expense. Anywhere on Broad Street for example.
The best way not to get poshed is to get the first round in. Then later in the night, when your compadres are drunk you can seagull a few drinks from them.
How to realise that you are getting poshed: when it dawns on you that the content of your wallet will either stretch to a curry and a taxi home, or one more drink - and you get the drink.
A popular place to get poshed is a distant cousin's wedding where the free bar has been withdrawn. Thus you end up drinking your taxi fare back to the B & B and having to scrounge a lift from some pissed-up arse in a Range Rover.
'Coming for a curry?'
'Nah, I'm totally poshed mate. Lend me a tenner?'
'Good night out?'
'Yeah, we went up town now I'm completely poshed. Lend me a tenner?'
'How was the wedding?'
'Crap, the freebies were denied so we got well poshed!'