1911 Encyclopædia Britannica/Nadir

NADIR (Arabic naḍīr, “opposite to,” used elliptically for naḍīr-es-semt, “opposite to the zenith”), a term used in astronomy for the point in the heavens exactly opposite to the zenith, the zenith and nadir being the two poles of the horizon. It is thus used figuratively of the lowest depth of a person’s spirits or the lowest point in a career.