The University got its nickname "Ole Miss" via a contest in 1897. That same year, the student yearbook was being published for the first time. As a way to find a name for the book, a contest was held to solicit suggestions from the student body. Elma Meek, a student at the time, submitted the winning entry of Ole Miss. Interestingly, the term "Ole Miss" is not derived from Mississippi, but was a term used by slaves for the wife of a plantation owner. This sobriquet was chosen not only for the yearbook, but also became the name by which the University is now affectionately known

cited from:

Chodas, Nadine. The Band Played Dixie: Race and Liberal Conscience at Ole Miss.