Etymology
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religious (adj.)

c. 1200, "devout, pious, imbued with or expressive of religious devotion," used of Christians, Jews, pagans; also "belonging to a religious order," from Anglo-French religius, Old French religious (12c., Modern French religieux) and directly from Latin religiosus, "pious, devout, reverencing or fearing the gods," also "religiously careful, anxious, or scrupulous," from religio "religious observance; holiness" (see religion).

The meaning "pertaining to religion" is from 1530s. The transferred sense of "scrupulous, exact, conscientious" is recorded from 1590s but restores or revives a sense right at home among the superstitious Romans. As a noun, from c. 1200 as "persons bound by vow to a religious order;" from late 14c. as "pious persons, the devout." Related: Religiousness.

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religiously (adv.)

late 14c., religiousli, "piously, devoutly, in a religious manner," from religious + -ly (2). Transferred sense of "exactly, strictly, scrupulously" is attested by 1570s.

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religieuse (n.)

"a nun, a religious woman," 1690s, from French religieuse, fem. of religieux "monk, religious person" (itself used in English from 1650s but much less common), noun use of the adjective meaning "religious" (see religious). As a type of pastry, attested from 1929.

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irreligious (adj.)

"not religious, without religious principles; condemning religion, impious, ungodly," c. 1400, from Late Latin irreligiosus "irreligious, impious," from assimilated form of in- "not, opposite of" (see in- (1)) + religiosus (see religious). Related: Irreligiously.

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religiosity (n.)

late 14c., religiosite, "religious feeling, reverence for God, piety," from Old French religiosete and directly from Late Latin religiositas "religiousness," from religiosus "pious, devout, reverencing or fearing the gods," also "religiously careful, anxious, or scrupulous" (see religious). In late 19c. especially "religious sentimentality, excessive susceptibility to religious emotion without corresponding regard for divine law."

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sacrosanct (adj.)

"superlatively sacred or inviolable," c. 1600, from Latin sacrosanctus "inviolable, protected by religious sanction, consecrated with religious ceremonies," from sacro, ablative of sacrum "religious sanction, religious rite" (from neuter singular of sacer "sacred") + sanctus, past participle of sancire "make sacred" (for both, see sacred). Earlier in partially Englished form sacro-seint (c. 1500).

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officiant (n.)

"one who conducts a religious service, one who administers a sacrament," 1836, from noun use of Medieval Latin officiantem (nominative officians) "performing religious services," present participle of officiare "to perform religious services," from Latin officium "a service; an official duty; ceremonial observance" (in Medieval Latin, "church service"); see office.

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apostate (adj.)
"unfaithful to a religious creed or to a principle," late 14c., see apostate (n.).
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Buddhism (n.)
"the religious system founded by the Buddha in India," 1801, from Buddha + -ism.
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