What is the meaning of the idiom "bad blood"?

The answer is enmity.

The idiom "bad blood" means "ill will or enmity between two people or groups." It is often used to describe a long-standing feud or disagreement that has resulted in feelings of hatred or resentment. For example, you might say "There's been bad blood between the two families for generations" or "The two countries have bad blood over a border dispute."

The other options are incorrect. "Seriously ill" is not the meaning of the idiom "bad blood." "Disgrace" means "shame or loss of reputation," and "troubles" means "problems or difficulties." While these words can be related to enmity, they are not the same thing.