Thread lazzing - to resurrect a long-dead thread (ref. Lazarus) for one of a number of reasons, usually to point and laugh at posters (usually long-gone) who predicted a 'soft landing'...
Winkle waving - Competitive posting e.g. between those who try to out-do someone else in how anti-establishment they are: e.g. poster 1 "I'd kill all FFers"; poster 2 "Traitor. I'd torture them then kill them, then kill their families, then I'd point at them and laugh"
Bum tickling - (a) a group of posters who knowingly wink and make in-jokes at each other e.g. Poster 1 "Oh yeah, you'd know all about cats wouldn't you..." Poster 2 "LOL"... (b) a group of posters who coalesce around a single, often older, poster, and literally cyber-tickle his anal passage.
Whumping - when someone starts a thread similar to or the same as a pre-existing thread, then the OPer of the existing thread replies to point out his thread. Toxic Avenger is particularly known for this.
Futtocking - Using a private group to arrange a pranking on a public thread e.g. "Let's all pretend to agree with punkfilth231 on his thread and then all realise we were wrong at the same time. Tee hee"...
Haircutting - Posters using terms like 'hey guys', 'killer apps', or other trans-atlantic nothingisms.
Tommying - Posting on a thread to demonstrate that you know literally everything there is to know and ever will be to know on that particular subject. Such posters are particularly prone to using the

emoticon.