'rivalrously' originates from the Latin word 'rivalis,' where 'rivalis' meant 'one using the same stream as another.'

'rivalis' transformed into the Old French word 'rival,' and eventually became the modern English word 'rivalrously' through Middle English.

Initially, it meant 'one who competes for the same resource,' but over time it evolved into its current meaning of 'in a manner that shows competition or rivalry.'