Wordorigins.org

When something is posh it is luxurious, stylish, pretentious, one of the good things in life associated with wealth and high social status. Its origin is not known for certain, but the best guess is that it comes from a 19th century slang term for money that in turn is taken from a Romani (Gypsy) word.

That Romani word is påš, meaning half. It’s used in various forms like the Welsh Romani påš xåra, or half-penny, or the Angloromani posh-hórri. The English slang use of posh to mean money appears as early as 1830 in the records of court proceedings. From the Sessions Papers of the Old Bailey from that year:

He had not got the posh (which means money) yet.