An Etymological Dictionary of Astronomy and Astrophysics

1a) To pour out, to spread in all directions.

1b) To spread by → diffusion.

2. Spread out, diffused, scattered, as → diffuse reflection.
   

See also:

→ diffuse atomic cloud, → diffuse galactic light, → diffuse interstellar band, → diffuse interstellar band carrier, → diffuse interstellar cloud, → diffuse interstellar medium, → diffuse molecular cloud, → diffuse nebula, → diffuse reflection, → diffuse transmission, → diffusion.

Etymology (EN): L. diffusus “spread, poured forth,” from dif- “apart, in every direction,” variant of → dis- + fuse, from fusus “melted, poured, cast,” p.p. of fundere
“to melt, cast, pour out,” from PIE *gheud-, from root *gheu- “to pour.”

Etymology (PE): Paxšidan “to diffuse, scatter, disperse,” infinitive of paxš “scattered, dispersed; withered, trodden,” (Manichean) Mid.Pers. pxš “to wither, fade; to grow ripe,” Proto-Iranian *paxš- “to cook,” cf. Av. pac- “to cook,” pacika- “cooked,” Mod.Pers. paz-, poxtan “to cook, bake,” Skt. pac- “to cook,” pakva- “ripe,” Gk. peptein “to cook, ripen,” L. coquere “to cook,” from which V.L. cocus “cook,” from which O.E. coc “cook;”
PIE *pek^w- “to cook, ripen;” paxšidé, p.p. of paxšidan.