CP can also mean a lot of other things (Captain Picard, Cheese Pizza, etc) but all of those things are references to child porn.
So, for instance, if you see a thread on 4chan and the title is, "Post all your Captain Picard," somebody will post child porn in that thread. Or cheese pizza. Or start screaming racial epithets, I don't know, shit, it's 4chan.
(Edit: I did a spit-take after going to the main forum after this and the first thing I saw was, "User CP.")