1
general::
The last extremity; also, death or ruin. For example, I'm supporting the union's demands to the bitter end, or Even though they fight a lot, I'm sure Mom and Dad will stay together to the bitter end. The source of this term may have been nautical, a bitter being a turn of a cable around posts, or bitts, on a ship's deck, and the bitter end meaning "the part of the cable that stays inboard." Thus, when a rope is paid out to the bitter end, no more remains. [Mid-1800s]
American Heritage Idioms