To be more posh than something else. Surprisingly, 'posher' isn't in the dictionary -yet- which could cause some confusion to whether or not it actually is a word.
Damn, Darren, you're so much posher than I am...
From the Yiddish abbreviation of Putz Knosher, or dick eater, Posher can mean a variety of things, from pussy to asshole to dick. All that matters is that you use it as a negative expression.
My Boss, Dave, is a real Posher for making me come in to work even though I had the day off.