Acronym - In Before The Spam
Generally posted on a forum after an announcement by somebody important and/or concerning something important, where everyone generally replies quickly with stupid shit.
Works similarly to IBTL.
Administrator: This is a very important announcement about this website/game/forum.
Person 1: IBTS!
Persons 2, 3 etc: OMG WTF STFU NO WAI
itty bitty titty society
that girl looks like she belongs to the IBTS