I've always thought it meant failing without any measure of success. Like Pluto Nash, that movie was a flop in every way. Failed to make a profit anywhere. Theater, DVD, and how often do you see Pluto Nash on TV? Not even a cult status, it was just considered bad all around and just couldn't make back it's budget. Firefly technically failed to be a hit TV show, but it's loyal following garnered it good DVD sales, a movie, and a legacy of just being a good sci-fi show. So technically, even though Firefly was canceled after one season, it's legacy lived and was able to at least get a movie out of that legacy.