Greek abyssos translates Hebrew tehom, the primordial deep of Genesis 1:2. In 1 Enoch it becomes the prison of the fallen Watchers; in Revelation, the pit under lock and key. English abyss kept all three jobs: the deep sea, the theological pit, and Nietzsche's gaze that returns.
Nietzsche's aphorism (Beyond Good and Evil, 1886) did more to populate the abyss than any theology: modern usage is 90 percent Nietzsche, 10 percent geology, and the pit of angels has become a motivational poster.
The older layer: tehom is cognate with Babylonian Tiamat, the salt-water goddess split to make the world. The abyss may be the corpse of a goddess, the deep as the body of what creation killed. Enoch's prison, in this reading, reuses the oldest crime scene.
Why did one word get assigned to geology, theology, and psychology all at once?