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.