A shortening of the proverb "better the devil you know than the devil you don't," meaning that, when forced to decide, it's better to choose a difficult or undesirable option that one is used to or familiar with rather than an unknown person or thing. A: "Why don't you just quit your job if you're so miserable?" B: "Well, I'm used to it at this point, and I worry that I'd feel the same way anywhere else. It's just the devil you know, I guess."