The רשב"ם writes he had an argument and convinced his grandfather רש"י the need to always stick to Pshat and not to
deviate at times to Drush.
The Pasuk says Miriam & Aaron spoke על אדות האשה הכשית All Meforshim explain that the כשית refers to Tzipora, Moishe's wife
The רשב"ם claims it couldn't refer toTzipora, being she was from מדין Bnei Keturah (Avraham) and כוש is from חם.
The רשב"ם writes based on an Agadda (legend) Moishe Rabeinu escaped Mitzrayim to כוש (Africa) He became king and married the queen of כוש. The רשב"ם claims she was the כשית that Aaron & Miriam spoke about.