from Urban Dictionary:
Thread Jacking:
Taking over a thread on a message board by taking a part of the original posted topic, twisting it around and "hijacking" the thread itself. What happens is that the original content contained in the post becomes moot and whatever the "Thread Jacker" has manipulated the content to be becomes the new content thereby "hijacking" the original intent of post. People now respond to the "thread jacker's" input and that becomes the focus of the thread.

Seems a bit verbose. Why not just leave it at 'thread hijacking'? I mean, everyone knows what it is. If members see it happening, they can just point it out and bring the thread back OT. If they don't notice it, then maybe the thread needed jacking.