I. “+ adjective
Etymology: Middle English, from Middle French, from Latin inconstant-, inconstans, from in- in- (I) + constant-, constans constant — more at constant
1. : marked by lack of constancy : likely to change frequently often without apparent or cogent reason : given to change of character, inclination, purpose, or location
unjust I may have been … but never inconstant — Jane Austen
2. obsolete : inconsistent
Synonyms:
fickle , capricious , mercurial , unstable : inconstant suggests a tendency to frequent change, often without good reason
for people seldom knew what they would be, young men especially, they are so amazingly changeable and inconstant — Jane Austen
it is often used in reference to persons incapable of steadfastness in love or in reference to changeable climatic and meteorological developments
supposing now … this lover of yours was not the sort of man we all take him to be, and that he was to turn out false, or inconstant — Anthony Trollope
places where the soil was fertile but the rainfall uncertain and the rivers shallow and inconstant — A.M.Schlesinger b.1888
fickle intensifies notions of pointless, even perverse, changeability and incapacity for steadfastness
she is fickle! How she turns from one face to another face — and smiles into them all — Edna S. V. Millay
but bitter experience soon taught him that lordly patrons are fickle and their favor not to be relied on — Aldous Huxley
the next morning was gay with fickle sun-showers; it was a harlequin day, a strayed reveler from April — Elinor Wylie
capricious is less derogatory than fickle but suggests motivation by caprice, whim, or fancy making for unexpected change
he seemed heartless and capricious, as ready to drop you as he had been to take you up — George du Maurier
the more capricious incidence of sexual passion — Lewis Mumford
the capricious severity of a mere despot — J.R.Green
a capricious and malevolent race of savages — Bernard De Voto
mercurial in this sense is likely to suggest changeability in mood, especially rapid rise from discouragement to mirth or elation, or to suggest a versatility of gifts
Allnutt's mercurial spirits could hardly help rising under the influence of Rose's persistent optimism — C.S.Forester
mercurial, euphoric, he could blaze into hectic social events and become a rather too brash and boyish “life and soul of the party” — Times Literary Supplement
unstable , a less colorful word, indicates an incapacity to remain stable or steady, with many changes and fluctuations
of some meddling, bold fanatic, mind unstable, weird, erratic — Sophia A. Jamieson
the occupation [of mining] in general is an unstable one — Lewis Mumford
the bolts of shade and flakes of light upon the countenances of the group changed shape and position endlessly. All was unstable, quivering as leaves, evanescent as lightning — Thomas Hardy
II. noun
: one that is inconstant