Brush up on 温习补习

Professor Oxbridge has been invited to give a speech in English. He hasn't studied English for so long that he'll have to brush up on it. That is, he'll have to refresh his knowledge of it.

Oxbridge教授已应邀用英语演讲。他很久没有学英语了，所以他不得不重新学习提高一下。也就是说，他必须重新温习一下他的知识。