pay (someone) back

To "pay someone back" means to return money that you borrowed from them. For example, when you have to borrow money from your roommate you can say:

I promise I'll pay you back by the end of the week.

Or, in situations like the one at the top of this post, it means to give someone money when they paid for an item that you both shared, like a lunch.

This phrase appears in these lessons: