Idiom of the week: Speaking of the devil

Speak of the devil... and he shall appear. Who hasn't ever said or heard it? 

"Speaking of the devil..." (short for the whole idiom) is used when a thing which is being discussed suddenly materializes or becomes real. Imagine you're about to set off for the weekend, everything's ready in your car, and you happen to lightly mention that last week, the battery in your friend's car died... And right there, right then, your own battery dies too! In a situation like this, you'd probably scream "Speaking of the devil!" (followed by a four-letter word)

The idiom may also be used when you're talking about someone, and suddenly, they appear. In fact, this seems to be the most popular usage.

"Speak of the devil... and he shall appear" is an old idiom which, in its early days, couldn't be used lightheartedly. Following superstition, it was dangerous to mention the Devil by name for fear that he might appear. It was only in the 19th century that the saying lost its power and popularized. 

In spite of its rocky way through history, "Speaking of the devil" is one of the most universal idioms nowadays. Interestingly enough, each language has a version of its own closely related to unique cultural and/or historical features. You may check this link to Wikipedia to learn more about it. Some languages speak of the devil, others speak "of a king", others speak "of a wolf", and some others even speak "of trolls"!