1. 激烈的；猛烈的；充满敌意的
    In a bitter argument or conflict, people argue very angrily or fight very fiercely.

    e.g. ...the scene of bitter fighting during the Second World War.
           第二次世界大战期间激烈的战斗场面
    e.g. ...a bitter attack on the Government's failure to support manufacturing...
           对政府支持制造业不力的猛烈抨击

2. 怨恨的；愤愤不平的
    If someone is bitter after a disappointing experience or after being treated unfairly, they continue to feel angry about it.

    e.g. She is said to be very bitter about the way she was sacked...
           据说，她对自己被以如此方式解雇而感到愤愤不平。
    e.g. His long life was marked by bitter personal and political memories.
           他漫长的一生充满了关于个人生活和政治生涯的苦涩记忆。

3. （经历或感觉）令人非常失望的，痛苦的，辛酸的
    A bitter experience makes you feel very disappointed. You can also use bitter to emphasize feelings of disappointment.

    e.g. I think the decision was a bitter blow from which he never quite recovered...
           我想这个决定对他是个沉重的打击，从此以后他一蹶不振。
    e.g. A great deal of bitter experience had taught him how to lose gracefully...
           大量惨痛的经历让他学会了如何坦然面对失败。

4. 寒冷的；刺骨的
    Bitter weather, or a bitter wind, is extremely cold.

bitter

    e.g. Outside, a bitter east wind was accompanied by flurries of snow.
           外面，刺骨的东风伴着阵阵小雪。
    e.g. ...after spending a night in the bitter cold.
           在严寒中度过一夜之后

5. （味道）苦的
      A bitter taste is sharp, not sweet, and often slightly unpleasant.

      e.g. The leaves taste rather bitter.
             这些叶子味道相当苦。
      e.g. ...as the wine ages, losing its bitter harshness, and becoming softer and smoother.
             年深日久，酒会褪去辣味，口感变得醇和起来

6. 苦啤酒
      Bitter is a kind of beer that is light brown in colour.

      e.g. ...a pint of bitter.
             一品脱苦啤酒

7. 坚持到底；拼到底
      If you say that you will continue doing something to the bitter end, especially something difficult or unpleasant, you are emphasizing that you will continue doing it until it is completely finished.

      e.g. The guerrillas would fight to the bitter end, he said, in order to achieve their main goal.
             他说，游击队员会为实现主要目标而战斗到底。

8. a bitter pill -> see pill