“sage” is a simple bulletin board feature that allows users to post a reply without bumping the thread. This was first introduced to 2channel as 'aburanuki' (油抜き, lit. “removing the oil”) ¹⁾ on December 2, 1999, then it was changed to 'sage' for 'sageru' (下げる, lit. “to lower”).

The feature simply requires the user to input 'sage' in the email field when posting. This will not work with any other variants of 'sage'. On 2channel, the use of 'sage' was encouraged to help threads maintain a relaxed atmosphere by keeping it away from the front page, presumably because trolls often target threads that appear on the front page,²⁾ plus it was the only way to ensure that you get sent back to the thread.³⁾

Unfortunately, this concept doesn't translate over to imageboards because all of the boards are paginated and there's no real incentive to use it, which makes it stick out like a sore thumb. In non-Japanese communities, some users treat 'sage' as a hostility-invoking trump card, often derailing threads into a stupid back-and-forth of bumping and not bumping, thus why 4chan made it invisible.⁴⁾

On the other hand, it has also been argued that the visibility of 'sage' was a way for these community to show what they deemed proper conduct, so hiding it had the effect of 4chan deteriorating into a 'free-for-all' which looked completely foreign to former users. However, a counterargument is that Gamergate and the 'culture war' obsession is the actual reason for this, but this explanation is just so boring and overdone.⁵⁾

• The antonym of 'sageru' (下げる) is 'ageru' (上げる), therefore the antonym of 'sage' is 'age'.
• In the early days of 4chan and on Kareha boards like 4-ch, using 'sage' on a board with IDs enabled would result in 'ID: Heaven' being the unique identifier.
• For some reason, it was only ever used properly on 4chan's /jp/ board.

• Sage on Niconico Encyclopedia
• Sage Progression on 2channel's Wiki