Etymology
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inconsistent (adj.)
1640s, "not agreeing in substance or form;" 1650s, "self-contradictory," from in- (1) "not, opposite of" + consistent. Related: Inconsistently.
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inconsistency (n.)
1640s, "something which is inconsistent;" 1650s as "quality of being inconsistent," from in- (1) "not, opposite of" + consistency. Related: Inconsistencies. Inconsistence (1630s) is marked "Now rare or Obs." in OED.
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schizophrenic (adj.)
1912; see schizophrenia + -ic. Also from 1912 as a noun, "schizophrenic person." Transferred adjectival sense of "contradictory, inconsistent" is by 1955.
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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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incongruous (adj.)
1610s, from Latin incongruus "incongruous, inconsistent," from in- "not, opposite of, without" (see in- (1)) + congruus "fit, suitable" (see congruent). Related: Incongruously; incongruousness.
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incongruence (n.)
c. 1600, from Late Latin incongruentia "incongruity," from incongruentem (nominative incongruens) "incongruous, inconsistent," from in- "not" (see in- (1)) + congruens (see congruent). Related: Incongruency.
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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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contradictory (adj.)

1530s, "mutually opposed, at variance, inconsistent, incapable of being true together," from Late Latin contradictorius "containing a contradiction or objection," from contradictus, past participle of contradicere "to speak against" (see contradiction).

Sense of "denying that something stated or approved is completely true" is from c. 1600. Meaning "fond of contradicting" is from 1891. Other adjectives, now obsolete, in the same sense were contradictorious (early 15c.), contradictious (c. 1600), contradictive (1620s). Related: Contradictorily. Used earlier as a noun (late 14c.) in plural contradictories, "a pair of propositions inconsistent with each other."

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paradoxical (adj.)

"of the nature of a paradox," 1580s, from paradox + -ical. Meaning "inconsistent with itself" is by 1630s. Competing forms were paradoxal (1560s), paradoxial (1620s), but these survive in niches, if at all. Related: Paradoxically.

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heterography (n.)
"incorrect spelling," 1783; see hetero- "other, different" + -graphy. Also "inconsistent but current spellings within a language, the use of the same letter with different value in different words or positions" (as English, in all ages), 1847.
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