What posters on a message boards will sometimes say in response to a thread/topic that will eventually be locked/closed. This is a way to show that the thread is incredibly dumb or goes against the rules of the message board. Used to sometimes get in a free post to raise one's post count before the topic gets locked, preventing any further posting in it.
Person 1 Posts: Topic-This Forum Sucks!
Person 2: In Before Lock!
Person 3(Moderator/Admin): This topic is locked for bashing the message board and being dumb.