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

Ill feeling

The correct answer is Ill feeling. The idiom "bad blood" means ill feeling or hostility between two people or groups. It is often used to describe a situation where there has been a disagreement or argument, and the two sides still feel angry or resentful towards each other.

The other options are incorrect. Not feeling well means feeling sick or unwell. Unhealthy means not in good health or condition. War means a state of armed conflict between two or more nations.