Paintball is a sport, in which players compete to eliminate opponents by hitting them with pellets containing paint (referred to as a paintball). Wiki
Online forums are discussion boards where people gather.
pbnation.com is the 14th largest forum on the internet and is dedicated to paintball, for example (pbnationdotcom)
A paintball poser is someone who owns some gear, has played before, looks forward to playing again...but whose forum time far exceeds his playing time.
They check the forums every day, post on them often, even the forums which are unrelated to them (unrelated even within their local paintball sphere). Yet, they never make it out to practice, or even attend recreational games.
They could be active on the forums every day, but only play paintball 2 or 3 times a year. When anyone asks in public they will say "yeah I'm a huge paintballer!" and show off their gun/equipment whenever someone visits.
Angers real paintballers who bleed sweat and cry on the field 2 - 3 days a week playing their hearts out. The real player comes home to see on the forums that they're being flamed or given advice by some paintball poser, overgrown mamas boy. The real player plays 20x the paintball the poser does, and the poser makes 20x the forum posts a real player does. Who's playing what sport exactly?
Meanwhile all cries of "just 1on1 me on the field then poser" are met with ignorance or escapism, as they really have very little prowess on the field at all.
Daniel: Man I can't believe I got hatered on on the forums...
Ray Tyler: Yeah that was uncalled for
Daniel: I can't stand hypocritical assholes who think they know what's better for me, but couldn't play the game to save their life.
Ray Tyler: Their forum posers don't let it get to ya...
Daniel: Their not just forum posers, their paintball posers!