'The Bitter End'

Apologies 'Bitters' end was as close as I could get !

The original phrase of 'the bitter end' is explained in old English by Captain Smith's - 'Seamen's Grammar' as:-

"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."

So basically the end of a cable or rope that stays wrapped around the ' Bit' on board the ship