“Janine, when am I going to get a chance to meet that new boyfriend of yours? Oh, speak of the devil, here he is now!”
“Speak of the devil.” is an old idiom that is derived from the superstition “Speak of the devil and he will appear.” In olden times people were afraid to say the name of the biblical devil (Satan, Lucifer, Beelzebub… he had many names) because they thought that would draw his attention and he would come after they said his name.
These days we use it to indicate that soon after we mention someone they suddenly show up. We know it is coincidence, but we use the idiom: “Speak of the devil” as if our conversation had some power. Since it is such a coincidence to mention someone and have them appear this phrase is usually said with some surprise.
“Speak of the devil, we were just talking about you!”
Although the phrase originates from calling the devil, an evil creature, we use it very neutrally these days. It doesn’t have to have a negative meaning. You will be able to tell from the tone of voice.
“Where is Gus today? Oh. Speak of the devil, you’re late again Gus!”
“I can’t wait until the postman gets here. Speak of the devil! I’m dying for my mail”