An Etymological Dictionary of Astronomy and Astrophysics

General: Delicate in texture, grain, or fiber.
Not bright or glaring.
Physics: Of a beam of particles or electromagnetic radiation, having relatively low energy, as opposed to → hard. → soft X-rays

Etymology (EN): M.E. softe “yielding, gentle, mild;” O.E. softe “gentle, easy;” cf. O.S. safti, O.H.G. semfti, Ger. sanft, M.Du. sachte, Du. zacht.

Etymology (PE): Narm “soft; smooth; mild,” from Mid.Pers. narm “soft; humble.”