Fickle
Fickle , adjective
[Old English fikel untrustworthy, deceitful, Anglo-Saxon ficol, from fic, gefic, fraud, deceit; compare fācen deceit, Old Saxon f{not transcribed}kn, Old High German feichan, Icelandic feikn portent. Compare Fidget.]
Not fixed or firm; liable to change; unstable; of a changeable mind; not firm in opinion or purpose; inconstant; capricious; as, Fortune's fickle wheel. — Shakespeare
They know how fickle common lovers are.