Found this on the Wikipedia page for the University:
The students of Stellenbosch University are nicknamed "Maties". The term probably arises from the Afrikaans word "tamatie" (meaning tomato, and referring to the old maroon colours of their Rugby Union player uniforms). However, some have postulated that the term comes from the Afrikaans colloquialism maat (meaning "buddy" or "mate") originally used diminutively by the students of the University of Cape Town's precursor, the South African College.