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late 14c., in astronomical sense, from Medieval Latin nadir, from Arabic nazir "opposite to," in nazir as-samt, literally "opposite of the zenith," from nazir "opposite" + as-samt "zenith" (see zenith). Transferred sense of "lowest point (of anything)" is first recorded 1793.
The point on the celestial sphere that is directly below the observer (90 degrees below the celestial horizon). Compare zenith.