Etymology
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obdurate (adj.)

mid-15c., "stubborn, inexorable, unyielding; hardened," especially against moral influences; "stubbornly wicked," from Latin obduratus "hardened," past participle of obdurare "harden, render hard; be hard or hardened; hold out, persist, endure," in Church Latin "to harden the heart against God," from ob "against" (see ob-) + durare "harden, render hard," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." Variant opturate is from early 15c. in medicine in a literal sense of "stopped, obstructed."  Related: Obdurately; obdurateness.

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obduration (n.)

c. 1400, obduracioun, "hard-heartedness; defiant impenitence," from Late Latin obdurationem (nominative obduratio) "a hardening," noun of state from past-participle stem of Latin obdurare "harden, render hard; be hard or hardened" (see obdurate).

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obeah (n.)

"sorcery, witchcraft" among Africans in Africa and the West Indies, 1760, from a West African word, such as Efik (southern Nigeria) ubio "a thing or mixture left as a charm to cause sickness or death," Twi ebayifo "witch, wizard, sorcerer."

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obedience (n.)

c. 1200, "the practice or virtue of submission to a higher power or authority;" late 14c., "dutiful compliance with a command or law," from Old French obedience "obedience, submission" (12c.), from Latin oboedientia "obedience," abstract noun from oboedientem (nominative oboediens) "obedient, compliant," present participle of oboedire "to obey" (see obey). In reference to dog training from 1930.

It has been a constant remark, that free countries have ever paid the heaviest taxes. The obedience of a free people to general laws, however hard they bear, is ever more perfect than that of slaves to the arbitrary will of a prince. [Alexander Hamilton to James Duane, Sept. 3, 1780]
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obedient (adj.)

c. 1200, "willing to serve (someone); willing to fulfill an obligation," from Old French obedient "obedient" (11c.), from Latin oboedientem (nominative oboediens) "obedient, compliant," present participle of oboedire "to obey" (see obey).

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obediently (adv.)

"in a compliant manner, dutifully," late 14c., from obedient + -ly (2).

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obeisance (n.)

late 14c., obeisaunce, "act or fact of obeying, submissiveness, quality of being compliant or dutiful; respectful submission, homage," from Old French obeissance "obedience, service, feudal duty" (13c.), from obeissant, present participle of obeir "to obey," from Latin oboedire "to obey" (see obey). The sense in English altered late 14c. to "bending or prostration of the body as a gesture of submission or respect, a bow or curtsy; deferential deportment; an act of reverence or deference" by influence of abase. Related: Obeisant.

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obelisk (n.)

"tapering rectangular stone column with a pyramidal apex," 1560s, from French obélisque (16c.) and directly from Latin obeliscus "obelisk, small spit," from Greek obeliskos "small spit, obelisk, leg of a compass," diminutive of obelos "a spit, pointed pillar, needle, broach; obelisk; bar of metal used as a coin or weight," a word of uncertain origin; according to Beekes, "clearly Pre-Greek." In printing, "a sign resembling a small dagger" (1580s). In dictionaries it is used to mark obsolete words. Greek obelos also was "a mark used in writing; horizontal line used as a diacritic." Related: Obeliscal; obeliskine.

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Oberon 

king of the faeries and husband of Titania in medieval lore, from French Obéron, from Old French Auberon, perhaps from a Germanic source related to elf. The satellite of Uranus of that name was discovered by William Herschel on Jan. 11, 1787, the same day he discovered the larger Uranian moon, Titania.

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obese (adj.)

"exceedingly fat," 1650s, back-formation from obesity and in part from Latin obesus "fat, stout, plump," literally "that has eaten itself fat," past participle of obedere "to eat all over, devour," from ob "about; because of" (see ob-) + edere "eat" (from PIE root *ed- "to eat"). According to OED, "Rare before 19th c." Related: Obeseness. Latin obesus was translated in Old English as oferfæt "overfat." As Latin obesus also could be read as "eaten up," it also was used in a passive sense, "wasted away, lean."

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