The word "bitter" describes a taste that is not sweet, but rather sharp or unpleasant. It can also describe a feeling of intense sadness, anger, or resentment towards someone or something. Full definition
They were kind of bitter tasting with no sweet flavor at all. (thecoconutmama.com)
Thus, the taste of bitter can be used to strengthen the most fundamental aspect of our health — the ability to extract the nutrients from our foods and nourish our bodies. (westonaprice.org)
This most often happens in bitter divorces, when one parent has strong resentment towards the other, etc.. (familyandcouplescounseling.com)