Acronym for "the internet is serious business," seen in many popular internet jokes. Used when people get too vehement or serious about their forum posts/discussions by making passive/aggressive ad hominem attacks.
"Take a chill pill dude, anyone with half a brain can see that Emilio Bonifacio is the worst 3B in the league judging by half a dozen sabermetrics that if you'd taken 30 seconds to research you would have seen yourself. I don't need to come down to your level to make this case to you."
"Dude, TIISB. Take it easy."