St. Thomas begins his Summa Contra Gentiles with a discussion on the nature and function of wisdom in human life. In Book I, Chapter 3, he explains the difference between faith and reason as means by which we determine what is true. On the reasonableness of accepting truths of faith in addition to truths of reason, he writes:
As therefore it would be the height of madness in a "plain man" to declare a philosopher's propositions false, because he could not understand them, so and much more would a man show exceeding folly if he suspected of falsehood a divine revelation given by the ministry of angels, on the mere ground that it was beyond the investigation of reason.
Put baldly, it's easy to agree that the opinion, "Whatever I don't understand can't be true," is irrational. But it can also be easy to fall into this opinion unwittingly, by not realizing that you don't understand something. "It's not what you don't know," as [insert homespun American humorist] put it, "but what you know that ain't so."1
And the easiest way to not realize you don't understand something may be when the thing you don't understand is expressed in words you use, but understand differently.
If, for example, someone says, "Aseity calls for nonisity," almost no one will seriously reply, "It most certainly does not!," because almost everyone will realize they have no idea what, if anything, that statement means. If, on the other hand, someone says, "Wives must be submissive to their husbands," a lot more people will with a lot more confidence reply, "They most certainly need not!"
The need to come to terms with someone -- to understand what he is saying before evaluating it -- is more evident when the term means nothing to me than when it means something. It's easy to forget to check that the term means the same to the person using it as it does to me, and the consequences are often me saying something idiotic.
1. The term "plain man" in the quotation from SCG translates the perhaps more expressive Latin word idiota, which literally means someone who can only speak his native language. Hence my custom of using "idiot" to mean "someone who talks about something without realizing he doesn't know what he's talking about."