Here's the scuttlebutt: Barge right in and swallow the anchor, and let's chew the fat and splice the main brace 'til we're three sheets to the wind. Listen, you son of a sea cook, I'm tired of minding my P's and Q's. I tell you, I'm all at sea, and this is the bitter end.
    -from the book description of
    When a Loose Cannon Flogs a Dead Horse There's the Devil to Pay: Seafaring Words in Everyday Speech by Olivia A. Isil

Learning the ropes

Many think that the phrase "Together till the bitter end" means that parting would be amidst the sweet sorrow of death including a miserably harsh ending. “People seem to love a sailor's yarn,” notes The Phrase Finder, “and anything with a whiff of the sea is seized on with enthusiasm. So much so that more thoughtful etymologists have dreamed up the inventive acronym CANOE - the Committee to Ascribe a Naval Origin to Everything. “The original phrase is a nautical term that has zilch to do with bitterness. It’s really the end of rope, line, or chain that is secured to the inboard of a ship. The posts or bollards where lines are wound are called bitts. Each wrap around the bollard is called a bitter with the last loop called—you guessed it—the bitter end. Good sailing requires avoiding letting the rope reach its bitter end because all the slack goes out of the line and there could be damage done to the ship in rough seas.

Today the phrase to the bitter end means “to the very end, in spite of harsh difficulties” or “to the limit of one's efforts - the last and direst extremity; to death itself, one who fights or holds out is called a bitter ender" and anyone who has the last word can be said to be employing bitter-enderism. The OED says that bitter is derived from the pre historic word biting as in "acrid-tasting," 725 AD at least, says The Phrase Finder. Sometime during the 14th the phrase shifted to the descibe a state of mind and was put to use in word like bittersweet. By 1849 the expression to the bitter end was in common use. In the appendix of John Bartlett’s (1820–1905) Familiar Quotations he notes:

    This phrase is nearly without meaning as it is used. The true phrase, “better end,” is used properly to designate a crisis, or the moment of an extremity. When in a gale a vessel has paid out all her cable, her cable has run out to the “better end,”—the end which is secured within the vessel and little used. Robinson Crusoe in describing the terrible storm in Yarmouth Roads says, “We rode with two anchors ahead, and the cables veered out to the better end.”

The bitter end was sometimes used as an instrument of punishment which may have given way to the darker meaning of today’s expression and while the naval origin does have a good case of CANOE, some say it isn't conclusive. A couple of etymologists cite Bible verses as a likely source. In Proverbs 5:4 where Solomon writes as Wisdom in his warnings against adultery he says, ”But her end is bitter as wormwood, sharp as a two-edged sword.” Solomon’s idea survives even to the harsh ending of Revelation and the trumpet of one of the seven angels when wormwood is identified as a "great star" that falls from heaven souring the waters. ”The third angel sounded his trumpet, and a great star, blazing like a torch, fell from the sky on a third of the rivers and on the springs of water— the name of the star is Wormwood. A third of the waters turned bitter, and many people died from the waters that had become bitter.” Common woodworm in a pungent herb of Eurasia and North Africa that has a bitter taste is used in making the liqueur absinthe.

Shakespeare uses the idiom numerous times in his plays and poems, as do many other dramatists and the earliest citation of the phrase in print comes from Captain John Smith founder of the British colony in Virginia when he published Sea Grammere in 1627 writing, "A Bitter is but the turne of a Cable about the Bits, and veare it out by little and little. And the Bitters end is that part of the Cable doth stay within boord."

The end of the rope

By the mid 1800’s the phrase was making the rounds in the political arena when the Congress. Globe published the following, “I am unfortunately among those who voted for the gentleman from Indiana, even ‘to the bitter end’” and “Our defence is a just one, and will be maintained by us to the ‘bitter end’.” By the roaring 20’s Lytton Strachey used the term to describe Baron Stockmar’s loyalty as a private adviser to Albert and Victoria saying that, “He would go on, working to the utmost and striving for the highest, to the bitter end." (Queen Victoria ) Three decades later English novelist Graham Greene wrote in his story about an accountant and his new wife, honeymooning in Monte Carlo when the couple's marriage suffers greatly as the lure of the casino becomes stronger than their devotion to each other, “A wife ought to believe in her husband to the bitter end.” (Loser Takes All) That’s all they wrote for now folks and while it’s the end of the line at least it’s not bitter!

Sources:

Common wormwood:
wordnet.princeton.edu/perl/webwn
Accessed October 29, 2005.

Kenneth G. Wilson (1923–). “bitter end” The Columbia Guide to Standard American English. 1993.

E. Cobham Brewer 1810–1897. “Bitter end (the)” Dictionary of Phrase and Fable. 1898.

John Bartlett (1820–1905). “Appendix” Familiar Quotations, 10th ed. 1919.

bitter end, The Phrase Finder:
http://www.phrases.org.uk/meanings/65800.htmlAccessed October 29, 2005.