Etymology
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plagal (adj.)

denoting a mode or melody in Gregorian music in which the final is in the middle of the compass instead of at the bottom, 1590s, from Medieval Latin plagalis, from plaga "the plagal mode," probably from plagius, from Medieval Greek plagios "plagal," in classical Greek "oblique," from plagos "side" (from PIE root *plak- (1) "to be flat").

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infallibility (n.)
"quality of being incapable of error," 1610s, from Medieval Latin infallibilitas, from infallibilis (see infallible).
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Servian 
1754 (n.), 1723 (adj.), from Medieval Latin Servia, from Serb Serb (see Serb).
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sexagesimal (adj.)
"pertaining to 60," 1680s, from Medieval Latin sexagesimalis, from Latin sexagesimus "the sixtieth," from sexaginta "sixty."
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febrile (adj.)
1650s, from Medieval Latin febrilis "pertaining to fever," from Latin febris "a fever" (see fever).
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heterogeneity (n.)
1640s, from heterogeneous + -ity, or else from Medieval Latin heterogeneitas, from heterogeneus.
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heraldic (adj.)
1772, on model of French héraldique (15c.), from Medieval Latin heraldus (see herald).
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Theobald 
masc. proper name, from Medieval Latin Theobaldus, from Old High German Theudobald, from theuda "folk, people" (see Teutonic) + bald "bold" (see bold). Form influenced in Medieval Latin by the many Greek-derived names beginning in Theo-.
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alchemist (n.)

1510s, from Old French alquemiste, from Medieval Latin alchimista, from Old French alchimie/Medieval Latin alkimia (see alchemy). Also see -ist. Earlier forms were alchemister (late 14c.), alkanamyer (late 15c.), with agent noun suffix -er.

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Ruthenian (adj.)

1850, of or pertaining to the western Ukrainian people (earlier Ruthene, 1540s), from Medieval Latin Rutheni "the Little Russians," a derivative of Russi (see Russia). For consonant change, compare Medieval Latin Prut(h)eni, from Prussi "Prussians." Another word in the same sense was Russniak.

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