Well yeah, BSD fans keeps on saying the Soukoku thing? And I've been able to get the meaning of it from one of the tumblr bloggers... So here's what I've read...

Asked by: Anonymous

who or what is soukoku? 

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Answered by: http://bungoustraydogz.tumblr.com/

I think I explained it before, but anyway for all those not familiar, soukoku (which literally means double black) was the name given to Dazai Osamu and Nakahara Chuuya as a pair. They were the most destructive in the mafia that one night of using "corruption" will be the end of organization's buildings. Mind you, soukoku is what canon calls them, and it is not a ship name, although it can also be used that way. Hope that clears things up!  

So, here's the photo of them before Dazai left the Port Mafia af.