Green’s Dictionary of Slang

smart, upper-class.

1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 199: A show-off, a snob, or a ‘poshy guy’.
2000		J. Hawes Dead Long Enough 9: Dramatically half lit in his poshy university office. [Ibid.] 101: Any sexy IRA men? said Jan in her most excruciating Loud Brit Poshy Girl voice.
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 58: Fockin poshie bastard.
2020		G. Armstrong Young Team 101: [B]ig poshy burds playin rugby.