Phrase used to imply that a forum thread is either stupid or pointless and is going to be locked by a forum mod/admin. It's a much cooler way of saying "I bet this thread is gonna be locked soon".
OP: Hey everyone, I thought I'd just bring up something that's been posted several times before...
OP+1: In before the lock! ;]
Forum Admin: I'm locking this thread because its stupid and pointless and has been brought up several times before.