sumō (相撲) (genre )
"The first sumō wrestlers were probably from the samurai or warrior class, about 1,000 years ago. It was remarkable for them to fight without weapons, wearing only a loincloth, instead of helmet and armour. Their unencumbered forms and quick movements led to comparisons with girls dancing gracefully. This form of combat, or ritual, was called suhada no mai, 'dancing naked,' suhada meaning 'naked,' no indicating possessive, and mai, 'Japanese dancing.' Suhada no mai came to be pronounced sumai and in the Heian Period it came to be smau, because of u-onbin. [Onbin a euphonic change or sound change in the middle or end of a word to make it easier to pronounce.] In the Heian Period sumō tournaments took place in the imperial court. Wrestlers came from all over Japan. There were few rules for sumō in those days. Wrestlers could punch and kick. Almost anything but pulling one's opponent's hair was allowed. The present, more extensive rules were refined in the Edo period."
Quoted from: Cultural Keys: The History of Japanese Words and Phrases by Hiroshi Otsuki and Bradley Grindstaff, p. 116.