{literally} ~volatile if/while mentioned.
{informal} said when somebody appears suddenly while he is being (or has just been) thought of or talked about.
Comp.: talk of the devil (and he is sure to appear).
* /А вот и сам виновник нашего спора пожаловал. Ничего не скажешь, лёгок на помине. — Here you are, the hero of our debates. There's nothing to be said — talk of the devil and he is sure to appear./
See also:
[speak of the devil and he appears]