Green’s Dictionary of Slang

posh adj.

[ety. unknown. The OED, like most modern authorities, rejects the trad. ‘port out, starboard home’ derivation. DSUE and J.P. Mayer (in Cohen (ed.), Studies in Slang I (1985), opt for a contraction of polished, well turned out, smart, sophisticated (note US milit. use in You Chirped a Chinful!! (c.1943), posh: bright and polished); M. Quinion in Port Out, Starboard Home (2004) prefers a link to posh n.2 ; the novelist P.G. Wodehouse uses push in 1903, which OED sees as a synon., but this may be linked to push n. (2a), a clique]

[20C+] (orig. milit.) smart, pertaining to the upper classes.

In derivatives

poshery (n.)

[1960s] upper-class airs.

In compounds

posh-knob (n.)

[2010s] a member of the upper class and/or establishment.

In phrases

all poshed up (adj.)

1. [1920s] dressed up.

2. rendered sophisticated, ‘classy’.

bit of posh (n.) [var. on bit of rough n.2 ]

[1970s+] an attractive young woman who is also considered intelligent or upper class.

do the posh (v.)

[1940s–50s] (Aus.) to spend to excess, to do something in style.

posh (up) (v.)

[1910s+] of a person, to smarten one’s clothes, house etc.