I agree it can be hard to put your finger exactly on what it is. My best sense of it is that it is something approximating your ability to follow through on so

I agree it can be hard to put your finger exactly on what it is. My best sense of it is that it is something approximating your ability to follow through on some task that a) was never, or has seized to be interesting or otherwise stimulating or b) is sufficiently difficult that many people would allow themselves to quit. Often related to the idea that there is some form of delayed gratification to be had.