The only free and comprehensive online etymological dictionary of the Spanish language
quejar (Verb) "to complain"
12th cent. From Vulgar Latin *quassiare 'id.,' from Latin quatere "to shake" (see cutir).
Indo-European
Romance
Portuguese queixar, French casser, Italian quassare
Corominas, J. & J. Pascual. Diccionario crítico etimológico castellano e híspanico. 6 vols. Madrid: Gredos. 1991-1997.
Dictionnaire Étymologique Roman. Online. Ed. by É. Buchi & W. Schweickard. 2018. http://www.atilf.fr/DERom.
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.