The phrase "fickle mistress" is an idiomatic expression that refers to something or someone that is unpredictable, changeable and difficult to please. It is often used to describe a person who has a tendency to be unfaithful or inconsistent in their loyalties, affections or behavior. The phrase is derived from the word "fickle," which means unstable or changeable in behavior, and the word "mistress," which refers to a woman who is the object of someone's affection or attention.