Speak of the devil is said when the person you were just discussing walks in. Speak of the devil, well, speak of the devil, and speak of the devil.
The full old form warns that he shall appear, and the superstition behind it was that naming the Devil summoned him. It is now entirely friendly and is one of the few pieces of superstition everybody still says.
1phrasespeak of the devilSaid when the person you were just discussing walks in.
“Well, speak of the devil! We were just talking about you, Tom.”
speak of the devil is a saying.
A shortening of speak of the devil and he shall appear, traceable to the seventeenth century.
an ellipsis of speak of the devil and he shall appear, which can be traced back to “talk of the Devil, and he’s presently at your elbow”, attested in 1666. The idea behind this (namely, that mentioning a dangerous creature may cause it to appear) is found in many cultures around the world; see taboo avoidance.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms