From God Didn’t Say That:
A proof text is a text that is used to demonstrate a point. This isn’t “proof” in the modern, scientific sense, though. The proof text doesn’t have to prove anything. And the proof text doesn’t even have to mean the same thing as what it’s demonstrating. The point of using a proof text was that it was considered better to use words of Scripture than to invent new ones — even if the words of Scripture were taken out of context.
The whole notion of text matching and of a proof text is generally foreign to our modern way of thinking. But it was central to how texts were understood 2,000 years ago.