Etymology
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inconstant (adj.)
c. 1400, "fickle, not steadfast," from Old French inconstant "variable, eccentric" (14c.), from Latin inconstantem (nominative inconstans) "changeable, fickle, capricious," from in- "not, opposite of, without" (see in- (1)) + constantem (see constant). Related: Inconstantly.
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inconstancy (n.)

1520s, of persons, "fickleness;" 1610s, of things, "mutability, irregularity," from Latin inconstantia "inconstancy, fickleness," abstract noun from inconstans "changeable, inconsistent" (see inconstant).

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inconstance (n.)

late 14c., inconstaunce, "changeableness in action, feeling, etc.; fickleness, unsteadiness," from Old French inconstance "inconstancy, instability" (13c.) and directly from Latin inconstantia "inconstancy, fickleness," abstract noun from inconstans "changeable, inconsistent" (see inconstant). In English, inconstancy is now the usual word.

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changeable (adj.)

mid-13c., "unstable, inconstant, unreliable," from Old French changeable "inconstant," from changier "to alter; exchange; to switch" (see change (v.)) + -able (see -able). Meaning "subject to variation" is from late 14c. Related: Changeably; changeability.

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changeful (adj.)

"inconstant, fickle," c. 1600, from change (n.) + -ful. Related: Changefulness.

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instable (adj.)
c. 1400, from Latin instabilis "unsteady, not firm, inconstant, fickle," from in- "not, opposite of" (see in- (1)) + stabilis (see stable (adj.)). Now mostly replaced by unstable.
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backlash (n.)
1815, of machinery, "reaction of wheels on each other produced by an inconstant load," from back (adj.) + lash (n.) "a blow, stroke." In metaphoric sense, it is attested from 1929.
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instability (n.)
early 15c., from Old French instabilité "inconstancy" (15c.) or directly from Latin instabilitatem (nominative instabilitas) "unsteadiness," from instabilis "unsteady, not firm, inconstant, fickle," from in- "not, opposite of" (see in- (1)) + stabilis (see stable (adj.)).
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inequality (n.)
early 15c., "difference of rank or dignity," from Old French inequalité (14c., Modern French inégalité) and directly from Medieval Latin inaequalitas, from Latin inaequalis "unequal, unlike, different (in size); changeable, inconstant," from in- "not, opposite of" (see in- (1)) + aequalis "equal" (see equal). In reference to magnitude, number, intensity, etc., from 1530s.
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courageous (adj.)
Origin and meaning of courageous

c. 1300, of persons, "valiant, brave, full of courage," also "desirous," from Anglo-French corageous, Old French corageus, corajos "eager, spirited, brave," also "capricious, inconstant" (12c., Modern French courageux), both on the notion of "following one's inner impulses," from corage "heart, innermost feelings; temper" (see courage). Of actions, speech, etc., "manifesting courage," by 1795. Related: Courageously; courageousness.

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