The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
bitter end
NOUN:
1. A final, painful, or disastrous extremity. 2.Nautical The inboard end of a chain, rope, or cable, especially the end of a rope or cable that is wound around a bitt.
ETYMOLOGY:
English bitter, bitt ( bitt + er1) + end. Sense 1, influenced by bitter.