From Takeourword.com Issue 89

This word comes ultimately from Arabic nador, and there are cognates in
Spanish, French, Portuguese and Italian nadir "opposite to". Its earliest
meaning in English was "a point in the heavens diametrically opposite to some
other point, especially to the sun." Chaucer used it in 1391: "The nadir of
the sonne is thilke degree Þat is opposit to the degree of the sonne, in the 7
signe, as thus, euery degree of aries bi ordre is nadir to euery degree of
libra by ordre." By the 15th century the word had taken on the meaning
"The point of the heavens diametrically opposite to the zenith; the point
directly under the observer." It is not until the late 18th century that we
find the word used in its figurative sense of "the lowest point (of anything);
the place or time of greatest depression or degradation," as in this quote
from Henry Hallam's History of Literature: "The seventh century is the
nadir of the human mind in Europe.