The only free and comprehensive online etymological dictionary of the Spanish language
rayo m. (Noun) "ray"
13th cent. From Latin radius 'id.' Of unknown origin.
Indo-European
Romance
Portuguese raio, Catalan raig, French rai, Italian raggio, Aromanian radzã, Romanian rază, Sardinian ragiu
Corominas, J. & J. Pascual. Diccionario crítico etimológico castellano e híspanico. 6 vols. Madrid: Gredos. 1991-1997.
Real Academia Española. Diccionario de la lengua española. Online. 23rd ed. Madrid: Espasa Calpa. 2014. http://www.rae.es/.
Sihler, A. New Comparative Grammar of Greek and Latin. Oxford University Press. 2008.