[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.
[1960s] upper-class airs.
[2010s] a member of the upper class and/or establishment.