It's an English saying so I said it in English, even if it's hard to tell with the magic translation.

[ Languages is one area in which L has the advantage, but Light can speak perfect English in addition to Japanese. It does make him wonder if L simply switches between languages all day. He can see L playing small games that no one will ever know about. ]

I don't really know the history of the phrase, but I think it used to be taken literally. A girl would go to a dance with a guy but end up more interested in somebody else. I know it happens in movies, but I don't know how often it was a real problem. Now, a lot of people use it to mean you should leave with the person you came with.

So, I came with you, and I'll leave with you.