The English word "nadir" originally had a strictly astronomical meaning the point of the celestial sphere which is opposite of the zenith. So, while the zenith is the highest point reached by a celestial object, the nadir is the lowest point reached by that object. The orientation of high and low are determined by gravitational pull; gravity pulls down towards the nadir. Since the late 18th century, however, the word has been used metaphorically to mean the lowest point of anything.
The word comes to English via medieval Latin
nadir, from Arabic
nazir, meaning "opposite." The complete Arabic astrological phrase is
nazir as-samt, "opposite the zenith." (English also gets the word "zenith" from Arabic; I'll post about that another time.)
The image below is from
Wikipedia:
