Etymology
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purebred (adj.)

also pure-bred, "of unmixed inheritance or ancestry," 1868, from pure (adj.) + bred.

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unsophisticated (adj.)
1620s, "unmixed," from un- (1) "not" + sophisticated (adj.). Meaning "ingenuous, natural, inexperienced" is recorded from 1660s.
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pureblood (adj.)

also pure-blood, "of unmixed inheritance or ancestry," 1851, from pure blood (n.), attested from 1751 in reference to breeding, from pure (adj.), which is attested from late 15c. in reference to unmixed descent or lineage, + blood (n.). As a noun meaning "a pure-blood animal" from 1882.

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sheer (adj.)
c. 1200, "exempt, free from guilt" (as in Sheer Thursday, the Thursday of Holy Week); later schiere "thin, sparse" (c. 1400), from Old English scir "bright, clear, gleaming; translucent; pure, unmixed," and influenced by Old Norse cognate scær "bright, clean, pure," both from Proto-Germanic *skeran (source also of Old Saxon skiri, Old Frisian skire, German schier, Gothic skeirs "clean, pure"), from PIE root *sker- (1) "to cut."

Sense of "absolute, utter" (sheer nonsense) developed 1580s, probably from the notion of "unmixed;" that of "very steep" (a sheer cliff) is first recorded 1800, probably from notion of "continued without halting." Meaning "diaphanous" is from 1560s. As an adverb from c. 1600.
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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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simplex (adj.)
"characterized by a single part," 1590s, from Latin simplex "single, simple, plain, unmixed, uncompounded," literally "onefold," from PIE compound of root *sem- (1) "one; as one, together with" + *plac- "-fold," from PIE root *plek- "to plait." The noun is attested from 1892, "simple uncompounded word."
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pure (adj.)

mid-13c., of gold, "unalloyed;" c. 1300 "unmixed, unadulterated; homogeneous," also "total, complete, absolute; bare, mere," also "sexually pure, virgin, chaste" (late 12c. as a surname, and Old English had purlamb "lamb without a blemish"), from Old French pur "pure, simple, absolute, unalloyed," figuratively "simple, sheer, mere" (12c.), from Latin purus "clean, clear; unmixed; unadorned; chaste, undefiled."

This is conjectured to be from PIE root *peue- "to purify, cleanse" (source also of Latin putus "clear, pure;" Sanskrit pavate "purifies, cleanses," putah "pure;" Middle Irish ur "fresh, new;" Old High German fowen "to sift").

It replaced Old English hlutor. The meaning "free from moral corruption" is recorded from mid-14c. In reference to bloodlines, attested from late 15c. In music, "mathematically perfect," by 1872.

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purity (n.)

c. 1200, purite, "freedom from moral contamination, sinlessness, innocence; righteousness; chastity," from Old French purete "simple truth," earlier purte (12c., Modern French pureté), from Late Latin puritatem (nominative puritas) "cleanness, pureness," from Latin purus "clean, pure, unmixed; chaste, undefiled" (see pure (adj.)). From mid-15c. as "freedom from admixture or adulteration."

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purblind (adj.)

c. 1300, pur blind "entirely blind," as a noun, "a blind person," later "partially blind, blind in one eye" (late 14c.), the main modern sense, from blind (adj.) + pur "entirely, completely, absolutely."

The adverb forming the first element is from an identical Middle English adjective pur "unadulterated, unmixed" (c. 1300), usually explained as from Old French pur and Latin purus (see pure). It might have been influenced by the Anglo-French perfective prefix pur- (see pur-). The sense of "dull" is recorded from 1530s.

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caste (n.)

"one of the hereditary social groups of India," 1610s from Portuguese casta "breed, race, caste," earlier casta raça, "unmixed race," from Latin castus "cut off, separated" (also "pure," via notion of "cut off" from faults), past participle of carere "to be cut off from," from PIE *kas-to-, from root *kes- "to cut." Caste system is first recorded 1840. An earlier, now-obsolete sense of caste in English is "a race of men" (1550s), from Latin castus "chaste."

Of the castes, the first three are the natural and gradually established divisions of the Aryan invaders and conquerors of India; the fourth was made up of the subjugated aborigines. The Sanskrit name for caste is varna, color, the different castes having been at first marked by differences of complexion, according to race, and in some degree according to occupation and consequent exposure. [Century Dictionary, 1895]
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