When someone takes your email thread from one topic completely off topic to some other subject.
Tim:
Email Subject: Snowboarding this weekend
Hey, anyone want to ride Hunter this weekend?

reply:
Will:
Email Subject: Snowboarding this weekend
Hey, I got a really cool macbook air, this thing rocks and kicks the ass of any Microsoft junk you tools use

Tim:
Email Subject: thread jacked MS vs Apple (was Snowboarding)
Damn I hate it when I get thread jacked.

by SoCal or bust March 19, 2008
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 the that becomes the focus of the tread.
LK25 did an excellent job of thread jacking Koko's post on Dennis Landolt.

Koko's posted topic on Dennis Landolt (PSU Football player) and comments regarding his development on Penn States FOS site becomes indictment of whole offensive line:

KokobewareHI: "Why do I feel like this thread is about to get hijacked? What happened to my original question?"

PSUnittany: "LK25, the thread was about Landolt & his progress...and (of course) you spin it into another "the staff is lazy at recruiting - we are doomed!" threads...It's so tiresome. Please link me to any threads that you don't hijack & ruin. Thank you."

OCCalif: "Although LK25 has jacked this thread to a degree, I feel his concerns are somewhat valid- we should be concerned about our offensive line."
by s6 October 3, 2006
An advertiser on Urban Dictionary that promotes ridiculous Nigger costumes as "mens wear". Also any clothing worn while jacking off.
I wouldn't even wear Jack Threads to masturbate. I prefer a wife beater and boxer shorts.
by wolfbait51 April 28, 2011