fickle
/ˈfi-kəl/
adjective
marked by lack of steadfastness, constancy, or stability : given to erratic changeableness
inconstant fickle capricious mercurial unstable mean lacking firmness or steadiness (as in purpose or devotion). inconstant implies an incapacity for steadiness and an inherent tendency to change. fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.