In before

From The LURKMORE Wiki

Posted to predict that an established meme will soon be posted in the thread, in the form of "In before [meme]." If the predicted meme is indeed posted, a reply of "In after [meme]." may follow. Slightly more rarely, the initial prediction may be replied to with "In with [meme]." or "In during [meme]." It can also be applied to tripfags who have predictable behavior and will often post in such thread that are relevant to their interest.

An alternative use of the "In before [meme]." is to prevent future posters from posting that meme, since no one likes to look like an idort.