Nkosi was always used as a term of respect to an older, more knowledgeable, person. Certainly within our farm when the farm laborers addressed my father or grandfather. I have adopted the term in remember to them I suppose. I also think it was a Swahili term.
According to Wikipedia
"Nkosi Sikelel' iAfrika"* is "Lord Bless Africa" in the
Xhosa language so presumably Nkosi means 'Lord'?
*and also the national anthems of Tanzania and South Africa.