The meaning of ITT (usually written in capitals) is “in this thread”.
It is mostly prevalent on message boards, chatrooms and forums, where people may spill their thoughts on various threads.
“ITT” is used just for that; to encourage users to examine a subject in a specific thread.
The history of “ITT” is woven together with message boards and online forums, so its origin can be traced back into the 1990’s, when these types of sites had emerged.
It was first defined on Urban Dictionary in 2003.
The acronym had spread onto several sites, during the 2000’s and 2010’s, such as 4chan, Reddit, or Something Awful.
One may find many applications to “ITT” from serious to sarcastic, cynical and humorous, as it appears in all sorts of threads on the web.