E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bitter End (The).
A outrance; with relentless hostility; also applied to affliction, as, she bore it to the bitter end, meaning to the last stroke of adverse fortune. All Thy waves have gone over me, but I have borne up under them to the bitter end. Here bitter end means the end of the rope. The bitter-end is a sea term meaning that part of the cable which is abaft the bitts. When there is no windlass the cables are fastened to bitts, that is, pieces of timber so called; and when a rope is payed out to the bitter-end, or to these pieces of timber, all of it is let out, and no more remains. However, we read in Prov. v. 4, Her end is bitter as wormwood, which, after all, may be the origin of the phrase.