The origin of this phrase is the doomier 'Speak of the devil and he is bound to appear'. This stems from the fact that some things are so frightening
, so terrifying, so evil beyond the grasp of human minds, that to speak their name aloud is to invite dire consequences
Apart from the fact that, in certain parts of the English countryside, the Black Forest in Germany and Norway this is literally true, the general modern meaning in this scientific age is 'If you start talking about something, it may well come to pass'.
For example, just this very minute some workmates of mine were wondering what had happened to another workmate who had been sent to the local shop to buy a variety of sweets. He had been gone a long time, but no sooner had my workmates wondered as to his whereabouts, the errand-boy returned. 'Speak of the devil', we said, 'and he is bound to appear'. 'Ph'nglui mglw'nfah Cthulhu R'lyeh wgah'nagl fhtagn!', we continued.