bitter: sharp and unpleasant to the taste, and by extension anything that brings grief or hardship
The root idea is the harsh taste that makes a person recoil, the opposite of sweet. Undrinkable water and food that turns the stomach are called bitter in this plain, physical way.
From there the word reaches into human experience. Grief, loss, and dread are all described as bitter: a bitter cry, a bitter day, a bitter end. When the pain is inward and personal, the Bible pairs the word with the word for life or self to make the fixed phrase "bitter of soul," a set expression for someone crushed by sorrow, resentment, or despair.
At its far edge the same word can describe a people as bitter in the sense of fierce or ruthless, the harshness now aimed outward at others rather than felt within. All of these grow from one seed: something that is hard to take, whether on the tongue, in the heart, or at another's hands.