The phrase "bad blood" is an idiomatic expression that means there is a long-standing feud, animosity or hostility between two parties. It can also refer to a situation where one party feels they have been wronged by another and harbors ill will towards them as a result. The term originated from the idea of blood being thicker than water meaning that family ties are stronger than any other relationship, so when there is conflict between two families it can be difficult for them to forgive or forget.