تعریف و معنای "(speak|talk) of the devil"

What is the origin of the idiom "speak of the devil" and when to use it?

The idiom "speak of the devil" originates from an older, superstitious belief that mentioning the devil's name or discussing the devil could attract his attention or presence. To drive away potential harm or bad luck, people would often follow such a mention with a phrase like "speak of the devil" as a way to acknowledge the potential consequences humorously or to ease any discomfort associated with the superstition. In modern usage, it serves as a casual way to remark on someone who has been mentioned just as they enter the conversation.