An Etymological Dictionary of Astronomy and Astrophysics

The point on the celestial sphere
blocked from view by Earth and diametrically opposite to the → zenith (سرسو).

Etymology (EN): From M.L. nadir, from Ar. nazir “opposite to,” contraction of nazir as-samt (نظیر‌السمت), literally “opposite of the zenith,” from nazir “opposite” + samt “zenith.”

Etymology (PE): Pâsu, literally “direction of the foot,” from pâ “foot, step” (from
Mid.Pers. pâd, pây; Khotanese fad; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + su “direction, side” (from Mid.Pers. sôk “direction, side”).