Stands for "Pretty Obvious Sexual Tension Involving Nasty Girl". "Nasty" can also mean Nice, New, Nefarious, or Neat. "Girl" can also be subbed for Guy. When, during a social encounter, there's a clear mutual attraction and desire for sexual intimacy that is never consummated.
"Dude, I should be banging right now because I was straight POSTING at that party"
"Did you see me POSTING up with that chick?"
"I just want to engage in some serious POSTING action tonight"