Bang Bang Con is a conference in New York City every year, organized by The Recurse Center. My friend Rafik, who I sailed from Sicily to Malta with on my boat in 2012, gave a talk there on making a floppy disk drive play MIDI files by ejecting back and forth at a certain speed. I went along for the ride, and enjoyed the conference enough that I was glad to have the shirt.
I mostly like this shirt because of the color and the fit, but the ambiguous exclamation is an added plus. The !! comes from the UNIX command, and is pronounced ‘Bang Bang’ by developers.