"Speaking of the devil."

Meaning: the person or thing that we are talking about appears or comes

Why does it mean that?

The original proverb is "Speak of the Devil and he shall appear". This means that if you talk about the Devil, then we will come, appear, or show up.

"Speaking of the Devil" is the short form and it is what is commonly used in modern times.

When do we use it?

We use this expression when we are talking about somebody or something that is not there, but then the person or thing suddenly appears or comes.

We usually use this when we are talking about a person who is not there, but then that person comes or appears while we are still talking about them.

How do we use it?

We use this as an exclamation by itself. We do not put it any longer sentence.

We can say it to the person that just appeared to tell them that we were just talking about them.

A) Ben just a promotion.
B) Really? That's great. Ben deserves it.
A) Speaking of the devil! Hey Ben. We were just talking about your promotion.
B) Congratulations.
A) Thanks.

We can also use it to let the person we are talking to know that the person we are discussing has just entered or come.

A) I think Mary is a terrible person.
B) I agree. Speaking of the devil. There she is.
A) Did she just come in?
B) Yes.
A) Let's leave. I don't want to talk to her.

It is important that you know English expressions and idioms if you want to have fluent English conversations with native English speakers or other advanced English speakers. Do not try to learn many expressions and idioms at one time. Instead, study a few of these free English lessons each week and learn English expressions and words well. This will help your English vocabulary improve steadily, and you will start speaking English like a native speaker.