poshyoudaoicibaDictYouDict[posh 词源字典]
posh: [20] Although it only appeared as recently as the early 20th century, posh is one of the oldest chestnuts of English etymology. The story got around that it was an acronym for port out, starboard home, an allusion to the fact that wealthy passengers could afford the more expensive cabins on the port side of the ships going out to India, and on the starboard side returning to Britain, which kept them out of the heat of the sun.

Pleasant as this story is, though, it has never been substantiated. Another possibility is that posh may be the same word as the now obsolete posh ‘dandy, swell’, a slang term current around the end of the 19th century. This too is of unknown origin, but it has been tentatively linked with the still earlier 19thcentury slang term posh ‘halfpenny’, hence broadly ‘money’, which may have come ultimately from Romany posh ‘half’.
[posh etymology, posh origin, 英语词源]
posh (adj.)youdaoicibaDictYouDict
by 1914 (1903 as push), of uncertain origin; no evidence for the common derivation from an acronym of port outward, starboard home, supposedly the shipboard accommodations of wealthy British traveling to India on the P & O Lines (to keep their cabins out of the sun); as per OED, see objections outlined in G. Chowdharay-Best, "Mariner's Mirror," Jan. 1971; also see here. More likely from slang posh "a dandy" (1890), from thieves' slang meaning "money" (1830), originally "coin of small value, halfpenny," possibly from Romany posh "half" [Barnhart].
The cavalryman, far more than the infantryman, makes a point of wearing "posh" clothing on every possible occasion -- "posh" being a term used to designate superior clothing, or articles of attire other than those issued by and strictly conforming to the regulations. [E. Charles Vivian, "The British Army From Within," London, 1914]