Most often, a band of little musical talent or significance whatsoever. Usually, the type of band which your one crazy, musically incompetent friend listens to and rips guitar tab from off the internet in an attempt to impress others who could not tell the difference between a Really Shit Band and the sound of their own head imploding from horrible sounds the rest of us are subjected to.
I kicked Mark in the balls and he sounded like a Really Shit Band.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.