1. Until the point of completion or conclusion, even though it may be difficult, unpleasant, or take a long time to reach. Possibly of nautical origin, referring to the "bitts" on a dock to which a ship's ropes are moored. I'm not really enjoying this book, but I always make a point of sticking with a novel till the bitter end.

2. To the final or most critical extremity, such as death or total defeat. We might not have a chance of winning today, but we have to give it our all until the bitter end! My father stayed beside my dying mother's bed till the bitter end.