The "bitter end" is the inboard end of an anchor line or chain secured to a ship’s structure (the bitts or chain locker). When paying out line, this represents the absolute last part available, signaling that no more cable can be let out. In an emergency, this end can be quickly cut or released, allowing the crew to dump the anchor and chain overboard.

Key Details About the Nautical Term:
Origin: The term derives from the bitts, which are sturdy wooden posts or metal blocks on a ship's deck used for securing lines.

The "Bitter" Pin: Traditionally, the very last link of the anchor cable is attached to a "bitter pin" or securing device inside the chain locker.

Nautical Significance: Reaching the "bitter end" means there is no more anchor line left to pay out.

Emergency Usage: Because the bitter end is secured in the cable locker (often with a quick-release mechanism), it is designed for rapid abandonment of the entire anchor system in a crisis.

Figurative Meaning: The nautical term has evolved into the idiom "to the bitter end," which refers to continuing to fight or endure, regardless of difficulty, until the very last moment or total defeat.