Pop quiz!
What are the first four words of Deuteronomy?
Okay, time's up.
Answer: "These are the words" (1:1).
And so we get the title… Deuteronomy? Oh wait, no. In Hebrew, the title of Deuteronomy is actually "Words." The book is named after the words Moses speaks to the people. It makes sense, if you think about it. Earlier books were all about myth (Genesis) or miracles (Exodus). Deuteronomy might recount those myths and miracles, but it's done through speech, not action. It's all about words.
So what does "Deuteronomy" mean? In Greek and Latin, the word means "second law." It's basically a mistranslation of this verse in the book:
When he has taken the throne of his kingdom, he shall have a copy of this law written for him in the presence of the levitical priests. (17:18)
When it was translated from Hebrew to Greek, someone misread the verse and translated "a copy of this law" to "second law" or "Deuteronomy." But hey, it kind of works. After all, Deuteronomy is a retelling of the laws in Exodus, Leviticus, and Numbers. It's a second look at the law.