From Japanese 西 (nishi, “west”), from the way the western or left side of the sumo ranking list is considered less prestigious, by analogy with the setting sun as opposed to the rising sun.
"nishi." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/nishi>.
nishi. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/nishi