Green’s Dictionary of Slang

in an aristocratic, upper-class manner.

1944		J. Maclaren-Ross ‘The Phantom of the Cookhouse’ in Bitten by the Tarantula (2005) 172: Got up posh, shirt front, black coat and all.
1956		J. Curtis Look Long Upon a Monkey 112: Think we’re muck because we ain’t had the education to talk posh.
1966		P. Willmott Adolescent Boys of East London (1969) 90: They talked posh, real English.
1974–5		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 116: Been talkin’ posh in Britsville that long I lost me Ausie accent I suppose.
1980		P.D. James Innocent Blood (1981) 177: Why do you speak posh?
1993		K. Lette Foetal Attraction (1994) 34: I can’t talk posh!