Etymology
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sincere (adj.)

1530s, "pure, unmixed," from French sincere (16c.), from Latin sincerus, of things, "whole, clean, pure, uninjured, unmixed," figuratively "sound, genuine, pure, true, candid, truthful," of uncertain origin. The ground sense seems to be "that which is not falsified." Meaning "free from pretense or falsehood" in English is from 1530s.

There has been a temptation to see the first element as Latin sine "without." But there is no etymological justification for the common story that the word means "without wax" (*sin cerae), which is dismissed out of hand by OED and others, and the stories invented to justify that folk etymology are even less plausible. Watkins has it as originally "of one growth" (i.e. "not hybrid, unmixed"), from PIE *sm-ke-ro-, from *sem- "one" (see same) + root of crescere "to grow" (from PIE root *ker- (2) "to grow"). De Vaan finds plausible a source in a lost adjective *caerus "whole, intact," from a PIE root meaning "whole."

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sincerely (adv.)
1530s, "correctly;" 1550s, "honestly," from sincere + -ly (2). As a subscription to letters, recorded from 1702.
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insincere (adj.)
1620s (implied in insincerely), from Latin insincerus "spoiled, corrupted; not genuine, not pure, adulterated," from in- "not" (see in- (1)) + sincerus "genuine, candid" (see sincere). Related: Insincerely.
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sincerity (n.)

early 15c., sincerite, "honesty, genuineness," from Old French sinceritie (early 16c., Modern French sincérité) and directly from Latin sinceritatem (nominative sinceritas) "purity, soundness, wholeness," from sincerus "whole, clean, uninjured," figuratively "sound, genuine, pure, true, candid, truthful" (see sincere).

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*ker- (2)

Proto-Indo-European root meaning "to grow."

It forms all or part of: accretion; accrue; cereal; Ceres; concrete; create; creation; creature; Creole; crescendo; crescent; crew (n.) "group of soldiers;" croissant; cru; decrease; Dioscuri; excrescence; excrescent; griot; increase; Kore; procerity; procreate; procreation; recreate; recreation; recruit; sincere.

It is the hypothetical source of/evidence for its existence is provided by: Greek kouros "boy," korē "girl;" Latin crescere "come forth, spring up, grow, thrive, swell," Ceres, goddess of agriculture, creare "to bring forth, create, produce;" Armenian serem "bring forth," serim "be born."

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candid (adj.)
1620s, "white, bright," from Latin candidum "white; pure; sincere, honest, upright," from candere "to shine," from PIE root *kand- "to shine." In English, metaphoric extension to "frank, honest, sincere" first recorded 1670s (compare French candide "open, frank, ingenuous, sincere"). Of photography, "unposed, informal" 1929. Related: Candidly; candidness.
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heart-felt (adj.)
also heartfelt, "profoundly felt, deep, sincere," 1734, from heart (n.) + past tense of feel (v.).
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unfeigned (adj.)
late 14c., "sincere, genuine, true, real," from un- (1) "not" + past participle of feign (v.).
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single-minded (adj.)
1570s, "sincere, honest" (a sense also in single-hearted); meaning "having a single aim or purpose" is from 1860. See single (adj.) + -minded. Related: Single-mindedly; single-mindedness.
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feal (adj.)
"faithful," 1560s, not found in Middle English but apparently from Old French feal "faithful, loyal, true, sincere," collateral form of feeil, from Latin fidelis "loyal" (see fidelity).
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