The phrase “to the bitter end” means: To the last extremity; to death or utter defeat.
This expression has a double meaning, but it is hardly likely that the poetic resemblance between the two meanings is anything more than chance.
That is, in the words of the famous Captain John Smith in his A Sea Grammar (1627): “A Bitter is but the turne of a Cable about the Bits, and veare it out (let it out) by little and little. And the Bitters end is that part of the Cable doth stay within boord.”
Or, as a later seaman put it, “When a chain or rope is paid out to the bitter end, no more remains to be let go”, when the end of the chain or rope reaches the bitts, obviously no more can be paid out.
But death, the end of life, has long been thought to be bitter, and it is no more than natural that, poetically, we should say that when one has come to the end of life, the “end of one’s rope,” that he has come “to the bitter end.”
It has been contended that nautical usage is, properly, “to the better end,” the end of the rope or chain which, being inboard, is little used. But the language of the sea does not substantiate this argument.