Most often, a band of little musical talent or significance whatsoever. Usually, the type of bandwhich 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 differencebetween 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.
A completely irrelevant word, most likely something which will end up with you being questioned about the level of your sanity. Also, would probably be an interesting word to call out in the middle of an important business discussion.
Bob: "So what do you think of this year's statistics?"
Bob: "You're an idiot."