speak of the devil
The person just mentioned has appeared, as in Why, speak of the devilthere's Jeannie. This expression is a shortening of the older Speak of the devil and he's sure to appear, based on the superstition that pronouncing the devil's name will cause his arrival on the scene. The figurative use was already explained in James Kelly's Scottish Proverbs (1721).
|speak of the devil|
|mention someone's name who just then appears|