inconstancy

When your friend promises to save you the last piece of gum but doesn't, you might curse her for her inconstancy. Inconstancy means changeability, not sticking to a predetermined course.

Inconstancy is a word often associated with love––if someone promises to love you forever in eighth grade but ends up falling for someone else in ninth, their behavior testifies to the inconstancy of their love. Or the inconstancy of youth. Or maybe just the inconstancy of love in general.

Definitions of inconstancy
1

n the quality of being changeable and variable

Synonyms:
changefulness
Antonyms:
constancy, stability
the quality of being enduring and free from change or variation
Types:
capriciousness, unpredictability
the quality of being guided by sudden unpredictable impulses
Type of:
changeability, changeableness
the quality of being changeable; having a marked tendency to change

n unfaithfulness by virtue of being unreliable or treacherous

Synonyms:
faithlessness, falseness, fickleness
Type of:
infidelity, unfaithfulness
the quality of being unfaithful