語源は不明だが、おそらくジプシーのposh、半分、半分、posh-kooroona、半分のクローネ、クローネと同じ語源。
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]