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"sage" - a way of passively making your comment not contribute to the 'bump count' of a thread when put in the options field of a post ('bump count' being how visible your post is on 4chan).
This means your comment doesn't bump an image or thread to the top of a board. Try it out in /VIP/ on threads about to die, and you'll see they won't jump to the top of /VIP/.
This way, you can contribute to a thread without others seeing the thread. This, in theory, means you can tactically sage threads to keep a conversation going between a couple of people without outsiders derailing your conversation with nonsensical posts.