An Etymological Dictionary of Astronomy and Astrophysics

1a) To draw, stretch, or open out, especially over a flat surface, as something rolled or folded (often followed by out).

1b) To stretch out or unfurl in the air, as folded wings, a flag, etc.

2. An act or instance of spreading; expansion, extension, or diffusion (Dictionary.com).

Etymology (EN): M.E. spreden, from O.E. sprædan “to spread, extend,” cf. Dan. sprede, O.Swed. spreda, M.Du. spreiden, O.H.G. and Ger. spreiten “to spread,” from PIE root *sper- “to strew.”

Etymology (PE): Gostardan “to spread; to diffuse, to expand,” from Mid.Pers. wistardan “to extend; to spread;” Proto-Iranian *ui.star-; Av. vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + Av. star- “to spread,” starati “spreads” (cf. Skt. star- “to spread out, extend, strew,”
strnati “spreads;” Gk. stornumi “I spread out,” strotos “spread, laid out;” L. sternere “to spread;” Ger.
Strahlung “radiation,” from strahlen “to radiate,” from Strahl “ray;” from M.H.G. strāle; from O.H.G. strāla “arrow,” stripe; PIE base *ster- “to spread”).