From Arabic 'nazir' (opposite), from 'nazir al-samt' (point opposite the zenith) — fragments of Arabic astronomy in English.
The point on the celestial sphere directly below an observer, diametrically opposite the zenith; figuratively, the lowest point of a situation or condition.
From French 'nadir,' from Arabic 'naẓīr al-samt' (نظير السمت), meaning 'opposite the direction (of the zenith),' literally 'counterpart of the path.' The word is an abbreviation of this longer phrase, with only 'naẓīr' (opposite, counterpart) surviving the translation. Like 'zenith,' 'nadir' entered European astronomical vocabulary through Latin
Nadir is the astronomical partner of zenith, and both words are fragments of longer Arabic phrases — 'samt al-raʾs' (path of the head) for zenith and 'naẓīr al-samt' (opposite of the path) for nadir — but medieval translators abbreviated each differently, obscuring their original connection.