Etymology
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indifferent (adj.)
late 14c., "unbiased, impartial, not preferring one to the other" (of persons), "alike, equal" (of things), from Old French indifferent "impartial" or directly from Latin indifferentem (nominative indifferens) "not differing, not particular, of no consequence, neither good nor evil," from in- "not, opposite of" (see in- (1)) + differens, present participle of differre "set apart" (see differ). Extended sense of "apathetic, no more inclined to one thing than to another" first recorded early 15c.; that of "neither good nor bad" is from 1530s, on notion of "neither more nor less advantageous," but since 17c. it has tended toward "rather bad."
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indifferently (adv.)
late 14c., "alike, equally; indiscriminately;" c.1400, "unconcernedly, carelessly;" early 15c., "impartially, without preferring one to the other;" from indifferent + -ly (2).
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indifference (n.)
mid-15c., "quality of being neither good nor bad, neutral quality," from Latin indifferentia "want of difference, similarity," abstract noun from indifferentem (see indifferent). From late 15c. as "lack of prejudice, impartiality;" from 1650s as "state of being apathetic." Meaning "comparative mediocrity, inexcellence"" is from 1864.
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*bher- (1)

Proto-Indo-European root meaning "to carry," also "to bear children."

It forms all or part of: Aberdeen; amphora; anaphora; aquifer; auriferous; bairn; barrow (n.1) "frame for carrying a load;" bear (v.); bearing; Berenice; bier; birth; bring; burden (n.1) "a load;" carboniferous; Christopher; chromatophore; circumference; confer; conference; conifer; cumber; cumbersome; defer (v.2) "yield;" differ; difference; differentiate; efferent; esophagus; euphoria; ferret; fertile; Foraminifera; forbear (v.); fossiliferous; furtive; indifferent; infer; Inverness; Lucifer; metaphor; odoriferous; offer; opprobrium; overbear; paraphernalia; periphery; pestiferous; pheromone; phoresy; phosphorus; Porifera; prefer; proffer; proliferation; pyrophoric; refer; reference; semaphore; somniferous; splendiferous; suffer; transfer; vociferate; vociferous.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bharati "he carries, brings," bhrtih "a bringing, maintenance;" Avestan baraiti "carries;" Old Persian barantiy "they carry;" Armenian berem "I carry;" Greek pherein "to carry," pherne "dowry;" Latin ferre "to bear, carry," fors (genitive fortis) "chance, luck," perhaps fur "a thief;" Old Irish beru/berim "I catch, I bring forth," beirid "to carry;" Old Welsh beryt "to flow;" Gothic bairan "to carry;" Old English and Old High German beran, Old Norse bera "barrow;" Old Church Slavonic birati "to take;" Russian brat' "to take," bremya "a burden," beremennaya "pregnant."

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adiaphorous (adj.)
"indifferent, non-essential, morally neither right nor wrong," 1630s, from Greek adiaphoros "not different; indifferent," from a- "not" (see a- (3)) + diaphoros "different."
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nonchalant (adj.)

also non-chalant, "indifferent, unconcerned, careless, cool," 1734, from French nonchalant "careless, indifferent," present participle of nonchaloir "be indifferent to, have no concern for" (13c.), from non- "not" (see non-) + chaloir "have concern for," ultimately from Latin calere "be hot" (from PIE root *kele- (1) "warm"). French chaland "customer, client" is of the same origin. Related: Nonchalantly.

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stoical (adj.)
early 15c. in reference to philosophers, from stoic + -al (2). Related: Stoically. From 1570s as "indifferent to pleasure or pain."
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anythingarian (n.)
"one indifferent to religious creeds, one 'that always make their interest the standard of their religion,'" 1704, originally dismissive, from anything on model of trinitarian, unitarian, etc.
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coolly (adv.)

1570s, "without haste or passion," from cool (adj.) + -ly (2). From 1610s as "without heat;" 1620s as "in an indifferent manner;" 1844 as "with quiet presumption or impudence."

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negligent (adj.)

late 14c., necligent, of persons, "remiss, indifferent to duty," from Old French negligent "careless, negligent" (13c.) and directly from Latin negligentem (nominative neglegens) "heedless, careless, unconcerned," present participle of neglegere "to neglect" (see neglect (v.)). Of action, conduct, etc., c. 1500. Related: Negligently.

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