>>41917047A slide thread is referencing the idea of a 'forum slide' theorized by an old internet document detailing tactics used by subversive groups. The idea is if there's a piece of information on a forum you don't want people to see, you and your friends post lots of new things to slide that thing down out of people's sight and get people to forget about it. In the context of 4chan, it just means people think that someone's posting a low-quality thread in order to cause whatever's on page 10 to get sent to the archive. In reality most low-quality threads are simply low-quality because the average 4channer has a room temperature IQ.