Informal/abusive term for Popstarz, a "gay alternative" club night running in London since 1995 and which nowadays attracts a varied crowd including indie kids, emo kids and other scene kids as well as older men and women.
It refers to the contingent (hopefully a minority) of slags who attend and the occasional, partial atmosphere of a meat market for college and university kids in which some sink, some swim and others feel as though chucks of flesh are ripped out of them.
Also derivative verb to popslag (see below).
"Are you popslagging tonight?"
"I'm not going to popslags, I can't be arsed with all the emoguys.net gayyage."