The term stems from the Afrikaans word "bruin" which means brown and the slang term "ous" which is a term used to define a a group of people. The term is not considered derogatory however, it should not be used in polite conversation.
Similar terms are used to describe South African persons of Indian descent (charous/chaarous - this term is not derogatory) as well as Black persons (peki ous/darkie ous - both terms are derogatory) and White persons (vet ous - slightly derogatory)
Person: "That bruin ou dances well."
When talking about someone
Person: "You know Tyrone? He's that bruin ou from Wentworth."
The term has been used in song by the band TRO who exclaimed; "Bruin ous, bruin ous, we are the main ous."
Which loosely translates to: "Us coloureds are people of note."