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]

(orig. milit.) smart, pertaining to the upper classes.

[Wodehouse Tales of St Austin’s 37: That waistcoat [...] being quite the most push thing of the sort in Cambridge].
[UK]F. Dunham diary 8 Mar. Long Carry (1970) 33: These were ‘posh’ places compared to the usual dugouts. [Ibid.] 13 Nov. 92: Little did we think [...] that we were leaving the Arras trenches for the last time. Things were so ‘posh’ and ‘cushy’ here.
[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 229: Posh: Smart. Spruce.
[UK]Film Fun 8 Sept. 1: Ollie came along a little later looking very posh indeed.
[UK]M. Harrison Reported Safe Arrival 131: We’re goin’ ter ’ave a posh feed at the bes’ hotel.
[UK]Wodehouse Mating Season 7: Haddock, though not as posh as he might be [...] foots the bills.
[UK]J. Osborne Look Back in Anger Act I: There are only two posh papers on a Sunday.
[UK]T. Keyes All Night Stand 212: Shall I get a Wisdom from Boots, or a fancy black one, with gold inlay, at a posh shop?
[UK]Sun. Times Mag. 12 Oct. 28: You imagine them in their posh houses, getting up to all kinds of things.
[US]T. Wolfe Bonfire of the Vanities 514: You do go on about the posh Mr. McCoy and his posh auto and his posh flat and his posh job and his posh daddy and his posh girlfriend.
[UK]D. Farson Never a Normal Man 128: The atmosphere was that of a posh Bedlam.
[Aus]P. Temple Black Tide (2012) [ebook] Bloody posh [...] S’pose this is where me sixty grand went.
[UK]M. Amis Experience 17: It used to be cool to be posh.

In derivatives

poshery (n.)

upper-class airs.

[UK]K. Williams Diaries 19 July 217: These young men in Hardy Amies suits who always have boxes of fifty fags and generate poshery.

In compounds

posh-knob (n.)

a member of the upper class and/or establishment.

[UK]K. Sampson Killing Pool 153: They [i.e. criminals] socialise with posh-knobs and soap stars and footballers.

In phrases

all poshed up (adj.)

1. dressed up.

[UK]N&Q 12 Ser. IX 347: Poshed Up. Dressed up for a special occasion.

2. rendered sophisticated, ‘classy’.

[UK]J. Sparks Burglar to the Nobility 160: I called it the Penguin Club [...] all poshed-up so the seedier members of the fraternity would feel ill-at-easem, but the spenders wouldn’t.
bit of posh (n.) [var. on bit of rough n.2 ]

an attractive young woman who is also considered intelligent or upper class.

[US]Ian Dury ‘Billericay Dickie’ [lyrics] A nice bit of posh from Burnham-on-Crouch.
[UK]Guardian G2 27 July 22: She’s sexy, she is. Bit of posh.
[UK]J.J. Connolly Layer Cake 49: First time out he’s got himself hitched up to this bitta posh who’s half a duchess.
[UK]Indep. Rev. 18 Feb. 9: Mags and Jan (Den’s bits of posh).
posh (up) (v.)

of a person, to smarten one’s clothes, house etc.

[UK](con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier 152: Posh Up.—To smarten one’s appearance.
[UK]F. Bason Diary I (1950) 75: [Somerset] Maugham’s daughter got married today. Mum and I poshed ourselves up and went and looked on.
[UK]C. Day Lewis Otterbury Incident 59: He had poshed himself up to call on Ted’s sister, Rose.
[UK]C. Harris Three-Ha’Pence to the Angel 103: May as well posh meself up even if it is only Flo and Jim.
[UK]N. Dunn Up the Junction 29: The club is an old cellar poshed up with hardboard and flashy paper.
[UK]Stage (London) 10 Jan. 16/4: He brought to it the touich of lively Yorkshire comedy (with no attempt to posh up the accent).
[Ire](con. 1945) S. McAughtry Touch and Go 80: Her accent was poshed up.