The Augmented 4th

There is no doubt about the status of this interval! It is unmistakably dissonant. In fact, it is usually considered the most dissonant of all the intervals. So much so that in the Middle Ages it was known as "the devil in music," and musicians were advised to avoid it.

we've all known an augmented fourth or two ...

damn, I love This American Life.

Source: Columbia University : Sonic Glossary