1. the quality of being changeable, unpredictable, or volatile in mood or behavior

Examples

• Her mercurialness made it difficult for friends to know what to expect from her.
• The stock market’s mercurialness kept investors on edge throughout the week.
• His mercurialness as a leader created uncertainty among his staff.
• The weather’s mercurialness in spring meant carrying both sunglasses and an umbrella.
• Critics noted the mercurialness of the artist’s creative output over the years.
• The mercurialness of public opinion made it hard to predict election outcomes.