An Etymological Dictionary of Astronomy and Astrophysics

Toward or in a lower physical position.
See also:

→ download, → downgrade, → upload, → downsizing, → meltdown, → overload, → spin-down, → top-down structure formation.

Etymology (EN): M.E. doun, from O.E. dune “downward,” short for adune, ofdune, from a-, of “off, from” + dune “hill.”

Etymology (PE): Pâyin “bottom, below; at the foot of,” from pâ(y) “foot; step;” Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-.
Foru “down, downward; below, under, beneath; low,” Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” Skt. pravát- “a sloping path, the slope of a mountain.”