leave a bitter taste in his mouth直译过来就是：“在他嘴里留下苦味”，这个短语的正确意思是：“给他留下不愉快的印象”。因此，当美国人说"I don't want to leave a bitter taste in his mouth."时，他/她要表达的意思就是:"I don't want to leave him a bad impression."、"I wouldn't like to leave him an unhappy memory."。
Shirley: You look pale today. Why don't you ask for a leave?
Jane: Well, I have to accompany one of our foremost clients to our factory. I don't want to leave a bitter taste in his mouth.
①You can ask the boss since you cannot make heads or tails of it.
②You can go back to have a rest and do it tomorrow.
③Don't worry. I will try my best to finish the mission.
④I will go to visit the client myself.
bitter adj. 苦的
As I said, babies love the taste of sweet and hate the taste of bitter.
Whether one's situation feels bitter or sweet is often subjective.
If you wish to be the best man, you must be prepared to suffer the bitterest of the bitter.