This is a word that has been around since the late 1800s, but is not used much these days, except for many small businesses which use it in their name. It means smart, elegant, or fashionable, such as posh clothes. It also means upper-class or genteel. It is also an herb.
A common reference is that back when ocean liners were the only way to cross the Atlantic, the preferred staterooms were those that faced “port out and starboard home.” As the ship crossed the North Atlantic, sunlight came into the room from the direction of the equator to the south. In a “posh” room you were on the port side (left) on the way south and east, and the starboard side (right) on the way home. Although this explanation is completely wrong, it makes for a good story.