From the old Norse word "bitt" or beam. A pair of posts fixed on the deck of a ship for securing lines. "Bitter" became a term for a single turn of a cable around the bitts, which was usually the very end of the rope. It became applied to a situation when a person was at the last extremity or very end of his resources. A parallel definition comes from the end of a rope that sometimes hangs over the side of a ship and is closest to the ocean. It's very end is "salty" or "bitter" since it often trails in the water.
