Etymology
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severity (n.)

late 15c. (Caxton), "austerity or strictness of life," from French severite, from Latin severitatem (nominative severitas) "seriousness, strictness, sternness," a derivative of severus "stern, strict, serious" (see severe). The meaning "strictness in dealing with others" is recorded from 1520s. OED marks severeness (1570s) as "rare."

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*segh- 
Proto-Indo-European root meaning "to hold."

It forms all or part of: Antioch; asseverate; asthenia; asthenosphere; cachectic; cachexia; calisthenics; cathexis; entelechy; eunuch; epoch; hectic; Hector; ischemia; myasthenia; neurasthenia; Ophiuchus; persevere; schema; schematic; scheme; scholar; scholastic; school (n.1) "place of instruction;" severe; severity; Siegfried.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sahate "he masters, overcomes," sahah "power, victory;" Avestan hazah "power, victory;" Greek skhema "figure, appearance, the nature of a thing," related to skhein "to get," ekhein "to have, hold; be in a given state or condition;" Gothic sigis, Old High German sigu, Old Norse sigr, Old English sige "victory."
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smartness (n.)
c. 1300, "severity," from smart (adj.) + -ness. From 1752 as "trimness," 1800 as "cleverness."
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extremely (adv.)
1530s, from extreme + -ly (2). Originally "with great severity," later more loosely, "in extreme degree" (1570s).
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oppressor (n.)

"one who exercises undue severity in the use of power or authority," c. 1400, oppressour, from Old French opresseor, from Latin oppressor "a crusher, a destroyer," from opprimere (see oppress (v.)). In Middle English also "a criminal; a rapist" (mid-15c.).

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tolerant (adj.)
1784, "free from bigotry or severity in judging others," from French tolérant (16c.), and directly from Latin tolerantem (nominative tolerans), present participle of tolerare "to bear, endure, tolerate" (see toleration). Meaning "able to bear (something) without being affected" is from 1879. Related: Tolerantly.
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intemperance (n.)
early 15c., "lack of restraint, excess," also of weather, "inclemency, severity," from Old French intemperance (14c.) and directly from Latin intemperantia "intemperateness, immoderation, excess" (as in intemperantia vini "immoderate use of wine"), from in- "not, opposite of" (see in- (1)) + temperantia "moderation, sobriety, discretion, self-control," from temperans, present participle of temperare "to moderate" (see temper (v.)).
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tolerance (n.)
early 15c., "endurance, fortitude" (in the face of pain, hardship, etc.), from Old French tolerance (14c.), from Latin tolerantia "a bearing, supporting, endurance," from tolerans, present participle of tolerare "to bear, endure, tolerate" (see toleration). Of individuals, with the sense "tendency to be free from bigotry or severity in judging other," from 1765. Meaning "allowable amount of variation" dates from 1868; and physiological sense of "ability to take large doses" first recorded 1875.
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third degree (n.)
"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony. Third degree as a measure of severity of burns (most severe) is attested from 1866, from French (1832); in American English, as a definition of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.
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acrimony (n.)

1540s, "quality of being sharp or pungent in taste," from French acrimonie or directly from Latin acrimonia "sharpness, pungency of taste," figuratively "acrimony, severity, energy," abstract noun from acer "sharp" (fem. acris), from PIE root *ak- "be sharp, rise (out) to a point, pierce") + -monia suffix of action, state, condition. Figurative extension to personal sharpness or bitterness is by 1610s.

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