Posh "smart, stylish, splendid, luxurious" has an interesting history. As Wordorigins says, it's probably from the word posh meaning "Money; spec. a halfpenny, a coin of small value." The OED says "the semantic development may thus have been either from ‘money’ to ‘moneyed, wealthy’, and hence to ‘upper-class’ and ‘smart, stylish, luxurious’".
Posh meaning "money" is a shortening of Welsh Romani påš xā̊ra "halfpenny". The second element xā̊ra "penny" is ultimately from German Heller "small coin", from Middle High German haller phenninc - haller from the name of the town of Hall in south-west Germany (now called Schwäbisch Hall), and phenninc meaning "penny" (OED).
The first element, påš, is from Romani paš "half", which is related to Sanskrit पार्श्व pārśva "side". This is from parśu "rib; curved knife, sickle" from Proto-Indo-European *perḱ- "rib, breast" (Pokorny 820). This is probably the source of Lithuanian pìršys "forepart of a horse's chest".