Fickle (adjective)

1. Given to sudden and unaccountable changes of mood or behavior.
2. Likely to change frequently, especially as regards one's loyalties, interests, or affection.
3. Unstable in one's affections or loyalty.

Origin:

Middle english: from old french fikele, based on latin feculus 'wavering, weak', from facere 'to do'. the word has been in use in english since the 14th century.

Examples:

1. He was a fickle lover, who would change his mind with the slightest encouragement from another woman.
2. Her fickle nature made it difficult for her to hold onto a job for more than a few months.
3. The stock market can be incredibly fickle, with prices fluctuating greatly from day to day.
4. She was always fickle about her opinions, never seeming to stick with one idea for very long.
5. Her friends would say she had a fickle heart, always falling in and out of love.