A phrase which is used when you're talking about someone behind that person's back, and that person appears on the scene.
An example
Peter and Randy are having a nice discussion about Ron:
Peter: "You know, I have never seen anyone with such a big, hairy stomach as Ron!"
Ron appears to the scene
Randy: "Speaking of the devil."
If Ron hears this, it might leave him
confused and maybe a bit hurt, wondering what Peter and Randy were saying about him.
Poor Ron. Of course it all depends on how Randy said it; if he had said it to Ron with a smile that says "I'm
just
kidding!", Ron would have
realized that Randy wasn't
serious.
BUT if Randy had said it to Peter with with an
icy tone in his voice,
his back
deliberately turned to Ron, then Ron would have, in case he heard it, been
aware that Peter and Randy were talking something
bad about him.
Bastards.