The Bitter End

[Bricker]

What is the bitter end? Why is it bitter?

Bitter? I hardly know 'er! Etc.

[Dead_Badger]

It’s originally a ropeworking term:

@Wikipedia

The bitter end
More a ropeworker’s term than a knot term, it refers to the end of a rope that is tied off, hence the expression “hanging on to the bitter end”. A bitt is a metal block with a crosspin used for tying lines to, found on docks. In fact the bitter end is the end of the Anchor “Cable” that connects to the Anchor Bitts in the cable locker under the forecastle or poop using the bitter pin. (British nautical usage). Other uses may be borrowed from this derivation.

[samclem]

…or, maybe not.

World Wide Words: Bitter end

Michael Quinion is one of the better linguists out there, and he writes from Britain. He suggests, in the link I"ve provided, that it might not be as clear as you think.

Well worth reading.